Merge branch 'features/optional-deps' into develop

This includes:
- Much better variant support (+debug/-debug)
- Optional dependency support (depends_on(... , when='<condition>')
- New config file format (YAML in ~/.spack)
- New Spec format (YAML in $prefix/.spack/spec.yaml)

53 files changed, 7547 insertions, 1188 deletions

diff --git a/lib/spack/docs/_themes/sphinx_rtd_theme/footer.html b/lib/spack/docs/_themes/sphinx_rtd_theme/footer.html
index 6347a440d7..d000dcbc2c 100644
--- a/lib/spack/docs/_themes/sphinx_rtd_theme/footer.html
+++ b/lib/spack/docs/_themes/sphinx_rtd_theme/footer.html
@@ -22,7 +22,12 @@
       {%- endif %}
     {%- endif %}
 
+    <br/>
+    Written by Todd Gamblin (<a href="mailto:tgamblin@llnl.gov">tgamblin@llnl.gov</a>) and
+    many contributors. LLNL-CODE-647188.
+
     {%- if last_updated %}
+      <br/>
       {% trans last_updated=last_updated|e %}Last updated on {{ last_updated }}.{% endtrans %}
     {%- endif %}
     </p>
@@ -33,4 +38,3 @@
   {%- endif %}
 
 </footer>
-
diff --git a/lib/spack/docs/conf.py b/lib/spack/docs/conf.py
index c2b2d0e37c..7303d7fef6 100644
--- a/lib/spack/docs/conf.py
+++ b/lib/spack/docs/conf.py
@@ -94,7 +94,7 @@ master_doc = 'index'
 
 # General information about the project.
 project = u'Spack'
-copyright = u'2013-2014, Lawrence Livermore National Laboratory'
+copyright = u'2013-2015, Lawrence Livermore National Laboratory.'
 
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the
@@ -203,7 +203,7 @@ html_last_updated_fmt = '%b %d, %Y'
 #html_show_sourcelink = True
 
 # If true, "Created using Sphinx" is shown in the HTML footer. Default is True.
-#html_show_sphinx = True
+#html_show_sphinx = False
 
 # If true, "(C) Copyright ..." is shown in the HTML footer. Default is True.
 #html_show_copyright = True
diff --git a/lib/spack/external/__init__.py b/lib/spack/external/__init__.py
index 1cc981930a..0578022210 100644
--- a/lib/spack/external/__init__.py
+++ b/lib/spack/external/__init__.py
@@ -28,6 +28,11 @@ packages that are included in spack.
 
 So far:
     argparse:    We include our own version to be Python 2.6 compatible.
-    pyqver2:     External script to query required python version of python source code.
-                 Used for ensuring 2.6 compatibility.
+
+    pyqver2:     External script to query required python version of
+                 python source code. Used for ensuring 2.6 compatibility.
+
+    functools:   Used for implementation of total_ordering.
+
+    yaml:        Used for config files.
 """
diff --git a/lib/spack/external/yaml/LICENSE b/lib/spack/external/yaml/LICENSE
new file mode 100644
index 0000000000..050ced23f6
--- /dev/null
+++ b/lib/spack/external/yaml/LICENSE
@@ -0,0 +1,19 @@
+Copyright (c) 2006 Kirill Simonov
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/lib/spack/external/yaml/README b/lib/spack/external/yaml/README
new file mode 100644
index 0000000000..c1edf13870
--- /dev/null
+++ b/lib/spack/external/yaml/README
@@ -0,0 +1,35 @@
+PyYAML - The next generation YAML parser and emitter for Python.
+
+To install, type 'python setup.py install'.
+
+By default, the setup.py script checks whether LibYAML is installed
+and if so, builds and installs LibYAML bindings.  To skip the check
+and force installation of LibYAML bindings, use the option '--with-libyaml':
+'python setup.py --with-libyaml install'.  To disable the check and
+skip building and installing LibYAML bindings, use '--without-libyaml':
+'python setup.py --without-libyaml install'.
+
+When LibYAML bindings are installed, you may use fast LibYAML-based
+parser and emitter as follows:
+
+    >>> yaml.load(stream, Loader=yaml.CLoader)
+    >>> yaml.dump(data, Dumper=yaml.CDumper)
+
+PyYAML includes a comprehensive test suite.  To run the tests,
+type 'python setup.py test'.
+
+For more information, check the PyYAML homepage:
+'http://pyyaml.org/wiki/PyYAML'.
+
+For PyYAML tutorial and reference, see:
+'http://pyyaml.org/wiki/PyYAMLDocumentation'.
+
+Post your questions and opinions to the YAML-Core mailing list:
+'http://lists.sourceforge.net/lists/listinfo/yaml-core'.
+
+Submit bug reports and feature requests to the PyYAML bug tracker:
+'http://pyyaml.org/newticket?component=pyyaml'.
+
+PyYAML is written by Kirill Simonov <xi@resolvent.net>.  It is released
+under the MIT license. See the file LICENSE for more details.
+
diff --git a/lib/spack/external/yaml/__init__.py b/lib/spack/external/yaml/__init__.py
new file mode 100644
index 0000000000..f977f46ba7
--- /dev/null
+++ b/lib/spack/external/yaml/__init__.py
@@ -0,0 +1,315 @@
+
+from error import *
+
+from tokens import *
+from events import *
+from nodes import *
+
+from loader import *
+from dumper import *
+
+__version__ = '3.10'
+
+try:
+    from cyaml import *
+    __with_libyaml__ = True
+except ImportError:
+    __with_libyaml__ = False
+
+def scan(stream, Loader=Loader):
+    """
+    Scan a YAML stream and produce scanning tokens.
+    """
+    loader = Loader(stream)
+    try:
+        while loader.check_token():
+            yield loader.get_token()
+    finally:
+        loader.dispose()
+
+def parse(stream, Loader=Loader):
+    """
+    Parse a YAML stream and produce parsing events.
+    """
+    loader = Loader(stream)
+    try:
+        while loader.check_event():
+            yield loader.get_event()
+    finally:
+        loader.dispose()
+
+def compose(stream, Loader=Loader):
+    """
+    Parse the first YAML document in a stream
+    and produce the corresponding representation tree.
+    """
+    loader = Loader(stream)
+    try:
+        return loader.get_single_node()
+    finally:
+        loader.dispose()
+
+def compose_all(stream, Loader=Loader):
+    """
+    Parse all YAML documents in a stream
+    and produce corresponding representation trees.
+    """
+    loader = Loader(stream)
+    try:
+        while loader.check_node():
+            yield loader.get_node()
+    finally:
+        loader.dispose()
+
+def load(stream, Loader=Loader):
+    """
+    Parse the first YAML document in a stream
+    and produce the corresponding Python object.
+    """
+    loader = Loader(stream)
+    try:
+        return loader.get_single_data()
+    finally:
+        loader.dispose()
+
+def load_all(stream, Loader=Loader):
+    """
+    Parse all YAML documents in a stream
+    and produce corresponding Python objects.
+    """
+    loader = Loader(stream)
+    try:
+        while loader.check_data():
+            yield loader.get_data()
+    finally:
+        loader.dispose()
+
+def safe_load(stream):
+    """
+    Parse the first YAML document in a stream
+    and produce the corresponding Python object.
+    Resolve only basic YAML tags.
+    """
+    return load(stream, SafeLoader)
+
+def safe_load_all(stream):
+    """
+    Parse all YAML documents in a stream
+    and produce corresponding Python objects.
+    Resolve only basic YAML tags.
+    """
+    return load_all(stream, SafeLoader)
+
+def emit(events, stream=None, Dumper=Dumper,
+        canonical=None, indent=None, width=None,
+        allow_unicode=None, line_break=None):
+    """
+    Emit YAML parsing events into a stream.
+    If stream is None, return the produced string instead.
+    """
+    getvalue = None
+    if stream is None:
+        from StringIO import StringIO
+        stream = StringIO()
+        getvalue = stream.getvalue
+    dumper = Dumper(stream, canonical=canonical, indent=indent, width=width,
+            allow_unicode=allow_unicode, line_break=line_break)
+    try:
+        for event in events:
+            dumper.emit(event)
+    finally:
+        dumper.dispose()
+    if getvalue:
+        return getvalue()
+
+def serialize_all(nodes, stream=None, Dumper=Dumper,
+        canonical=None, indent=None, width=None,
+        allow_unicode=None, line_break=None,
+        encoding='utf-8', explicit_start=None, explicit_end=None,
+        version=None, tags=None):
+    """
+    Serialize a sequence of representation trees into a YAML stream.
+    If stream is None, return the produced string instead.
+    """
+    getvalue = None
+    if stream is None:
+        if encoding is None:
+            from StringIO import StringIO
+        else:
+            from cStringIO import StringIO
+        stream = StringIO()
+        getvalue = stream.getvalue
+    dumper = Dumper(stream, canonical=canonical, indent=indent, width=width,
+            allow_unicode=allow_unicode, line_break=line_break,
+            encoding=encoding, version=version, tags=tags,
+            explicit_start=explicit_start, explicit_end=explicit_end)
+    try:
+        dumper.open()
+        for node in nodes:
+            dumper.serialize(node)
+        dumper.close()
+    finally:
+        dumper.dispose()
+    if getvalue:
+        return getvalue()
+
+def serialize(node, stream=None, Dumper=Dumper, **kwds):
+    """
+    Serialize a representation tree into a YAML stream.
+    If stream is None, return the produced string instead.
+    """
+    return serialize_all([node], stream, Dumper=Dumper, **kwds)
+
+def dump_all(documents, stream=None, Dumper=Dumper,
+        default_style=None, default_flow_style=None,
+        canonical=None, indent=None, width=None,
+        allow_unicode=None, line_break=None,
+        encoding='utf-8', explicit_start=None, explicit_end=None,
+        version=None, tags=None):
+    """
+    Serialize a sequence of Python objects into a YAML stream.
+    If stream is None, return the produced string instead.
+    """
+    getvalue = None
+    if stream is None:
+        if encoding is None:
+            from StringIO import StringIO
+        else:
+            from cStringIO import StringIO
+        stream = StringIO()
+        getvalue = stream.getvalue
+    dumper = Dumper(stream, default_style=default_style,
+            default_flow_style=default_flow_style,
+            canonical=canonical, indent=indent, width=width,
+            allow_unicode=allow_unicode, line_break=line_break,
+            encoding=encoding, version=version, tags=tags,
+            explicit_start=explicit_start, explicit_end=explicit_end)
+    try:
+        dumper.open()
+        for data in documents:
+            dumper.represent(data)
+        dumper.close()
+    finally:
+        dumper.dispose()
+    if getvalue:
+        return getvalue()
+
+def dump(data, stream=None, Dumper=Dumper, **kwds):
+    """
+    Serialize a Python object into a YAML stream.
+    If stream is None, return the produced string instead.
+    """
+    return dump_all([data], stream, Dumper=Dumper, **kwds)
+
+def safe_dump_all(documents, stream=None, **kwds):
+    """
+    Serialize a sequence of Python objects into a YAML stream.
+    Produce only basic YAML tags.
+    If stream is None, return the produced string instead.
+    """
+    return dump_all(documents, stream, Dumper=SafeDumper, **kwds)
+
+def safe_dump(data, stream=None, **kwds):
+    """
+    Serialize a Python object into a YAML stream.
+    Produce only basic YAML tags.
+    If stream is None, return the produced string instead.
+    """
+    return dump_all([data], stream, Dumper=SafeDumper, **kwds)
+
+def add_implicit_resolver(tag, regexp, first=None,
+        Loader=Loader, Dumper=Dumper):
+    """
+    Add an implicit scalar detector.
+    If an implicit scalar value matches the given regexp,
+    the corresponding tag is assigned to the scalar.
+    first is a sequence of possible initial characters or None.
+    """
+    Loader.add_implicit_resolver(tag, regexp, first)
+    Dumper.add_implicit_resolver(tag, regexp, first)
+
+def add_path_resolver(tag, path, kind=None, Loader=Loader, Dumper=Dumper):
+    """
+    Add a path based resolver for the given tag.
+    A path is a list of keys that forms a path
+    to a node in the representation tree.
+    Keys can be string values, integers, or None.
+    """
+    Loader.add_path_resolver(tag, path, kind)
+    Dumper.add_path_resolver(tag, path, kind)
+
+def add_constructor(tag, constructor, Loader=Loader):
+    """
+    Add a constructor for the given tag.
+    Constructor is a function that accepts a Loader instance
+    and a node object and produces the corresponding Python object.
+    """
+    Loader.add_constructor(tag, constructor)
+
+def add_multi_constructor(tag_prefix, multi_constructor, Loader=Loader):
+    """
+    Add a multi-constructor for the given tag prefix.
+    Multi-constructor is called for a node if its tag starts with tag_prefix.
+    Multi-constructor accepts a Loader instance, a tag suffix,
+    and a node object and produces the corresponding Python object.
+    """
+    Loader.add_multi_constructor(tag_prefix, multi_constructor)
+
+def add_representer(data_type, representer, Dumper=Dumper):
+    """
+    Add a representer for the given type.
+    Representer is a function accepting a Dumper instance
+    and an instance of the given data type
+    and producing the corresponding representation node.
+    """
+    Dumper.add_representer(data_type, representer)
+
+def add_multi_representer(data_type, multi_representer, Dumper=Dumper):
+    """
+    Add a representer for the given type.
+    Multi-representer is a function accepting a Dumper instance
+    and an instance of the given data type or subtype
+    and producing the corresponding representation node.
+    """
+    Dumper.add_multi_representer(data_type, multi_representer)
+
+class YAMLObjectMetaclass(type):
+    """
+    The metaclass for YAMLObject.
+    """
+    def __init__(cls, name, bases, kwds):
+        super(YAMLObjectMetaclass, cls).__init__(name, bases, kwds)
+        if 'yaml_tag' in kwds and kwds['yaml_tag'] is not None:
+            cls.yaml_loader.add_constructor(cls.yaml_tag, cls.from_yaml)
+            cls.yaml_dumper.add_representer(cls, cls.to_yaml)
+
+class YAMLObject(object):
+    """
+    An object that can dump itself to a YAML stream
+    and load itself from a YAML stream.
+    """
+
+    __metaclass__ = YAMLObjectMetaclass
+    __slots__ = ()  # no direct instantiation, so allow immutable subclasses
+
+    yaml_loader = Loader
+    yaml_dumper = Dumper
+
+    yaml_tag = None
+    yaml_flow_style = None
+
+    def from_yaml(cls, loader, node):
+        """
+        Convert a representation node to a Python object.
+        """
+        return loader.construct_yaml_object(node, cls)
+    from_yaml = classmethod(from_yaml)
+
+    def to_yaml(cls, dumper, data):
+        """
+        Convert a Python object to a representation node.
+        """
+        return dumper.represent_yaml_object(cls.yaml_tag, data, cls,
+                flow_style=cls.yaml_flow_style)
+    to_yaml = classmethod(to_yaml)
+
diff --git a/lib/spack/external/yaml/composer.py b/lib/spack/external/yaml/composer.py
new file mode 100644
index 0000000000..06e5ac782f
--- /dev/null
+++ b/lib/spack/external/yaml/composer.py
@@ -0,0 +1,139 @@
+
+__all__ = ['Composer', 'ComposerError']
+
+from error import MarkedYAMLError
+from events import *
+from nodes import *
+
+class ComposerError(MarkedYAMLError):
+    pass
+
+class Composer(object):
+
+    def __init__(self):
+        self.anchors = {}
+
+    def check_node(self):
+        # Drop the STREAM-START event.
+        if self.check_event(StreamStartEvent):
+            self.get_event()
+
+        # If there are more documents available?
+        return not self.check_event(StreamEndEvent)
+
+    def get_node(self):
+        # Get the root node of the next document.
+        if not self.check_event(StreamEndEvent):
+            return self.compose_document()
+
+    def get_single_node(self):
+        # Drop the STREAM-START event.
+        self.get_event()
+
+        # Compose a document if the stream is not empty.
+        document = None
+        if not self.check_event(StreamEndEvent):
+            document = self.compose_document()
+
+        # Ensure that the stream contains no more documents.
+        if not self.check_event(StreamEndEvent):
+            event = self.get_event()
+            raise ComposerError("expected a single document in the stream",
+                    document.start_mark, "but found another document",
+                    event.start_mark)
+
+        # Drop the STREAM-END event.
+        self.get_event()
+
+        return document
+
+    def compose_document(self):
+        # Drop the DOCUMENT-START event.
+        self.get_event()
+
+        # Compose the root node.
+        node = self.compose_node(None, None)
+
+        # Drop the DOCUMENT-END event.
+        self.get_event()
+
+        self.anchors = {}
+        return node
+
+    def compose_node(self, parent, index):
+        if self.check_event(AliasEvent):
+            event = self.get_event()
+            anchor = event.anchor
+            if anchor not in self.anchors:
+                raise ComposerError(None, None, "found undefined alias %r"
+                        % anchor.encode('utf-8'), event.start_mark)
+            return self.anchors[anchor]
+        event = self.peek_event()
+        anchor = event.anchor
+        if anchor is not None:
+            if anchor in self.anchors:
+                raise ComposerError("found duplicate anchor %r; first occurence"
+                        % anchor.encode('utf-8'), self.anchors[anchor].start_mark,
+                        "second occurence", event.start_mark)
+        self.descend_resolver(parent, index)
+        if self.check_event(ScalarEvent):
+            node = self.compose_scalar_node(anchor)
+        elif self.check_event(SequenceStartEvent):
+            node = self.compose_sequence_node(anchor)
+        elif self.check_event(MappingStartEvent):
+            node = self.compose_mapping_node(anchor)
+        self.ascend_resolver()
+        return node
+
+    def compose_scalar_node(self, anchor):
+        event = self.get_event()
+        tag = event.tag
+        if tag is None or tag == u'!':
+            tag = self.resolve(ScalarNode, event.value, event.implicit)
+        node = ScalarNode(tag, event.value,
+                event.start_mark, event.end_mark, style=event.style)
+        if anchor is not None:
+            self.anchors[anchor] = node
+        return node
+
+    def compose_sequence_node(self, anchor):
+        start_event = self.get_event()
+        tag = start_event.tag
+        if tag is None or tag == u'!':
+            tag = self.resolve(SequenceNode, None, start_event.implicit)
+        node = SequenceNode(tag, [],
+                start_event.start_mark, None,
+                flow_style=start_event.flow_style)
+        if anchor is not None:
+            self.anchors[anchor] = node
+        index = 0
+        while not self.check_event(SequenceEndEvent):
+            node.value.append(self.compose_node(node, index))
+            index += 1
+        end_event = self.get_event()
+        node.end_mark = end_event.end_mark
+        return node
+
+    def compose_mapping_node(self, anchor):
+        start_event = self.get_event()
+        tag = start_event.tag
+        if tag is None or tag == u'!':
+            tag = self.resolve(MappingNode, None, start_event.implicit)
+        node = MappingNode(tag, [],
+                start_event.start_mark, None,
+                flow_style=start_event.flow_style)
+        if anchor is not None:
+            self.anchors[anchor] = node
+        while not self.check_event(MappingEndEvent):
+            #key_event = self.peek_event()
+            item_key = self.compose_node(node, None)
+            #if item_key in node.value:
+            #    raise ComposerError("while composing a mapping", start_event.start_mark,
+            #            "found duplicate key", key_event.start_mark)
+            item_value = self.compose_node(node, item_key)
+            #node.value[item_key] = item_value
+            node.value.append((item_key, item_value))
+        end_event = self.get_event()
+        node.end_mark = end_event.end_mark
+        return node
+
diff --git a/lib/spack/external/yaml/constructor.py b/lib/spack/external/yaml/constructor.py
new file mode 100644
index 0000000000..8c0ec181b2
--- /dev/null
+++ b/lib/spack/external/yaml/constructor.py
@@ -0,0 +1,678 @@
+
+__all__ = ['BaseConstructor', 'SafeConstructor', 'Constructor',
+    'ConstructorError']
+
+from error import *
+from nodes import *
+
+import datetime
+
+import binascii, re, sys, types
+
+class ConstructorError(MarkedYAMLError):
+    pass
+
+class BaseConstructor(object):
+
+    yaml_constructors = {}
+    yaml_multi_constructors = {}
+
+    def __init__(self):
+        self.constructed_objects = {}
+        self.recursive_objects = {}
+        self.state_generators = []
+        self.deep_construct = False
+
+    def check_data(self):
+        # If there are more documents available?
+        return self.check_node()
+
+    def get_data(self):
+        # Construct and return the next document.
+        if self.check_node():
+            return self.construct_document(self.get_node())
+
+    def get_single_data(self):
+        # Ensure that the stream contains a single document and construct it.
+        node = self.get_single_node()
+        if node is not None:
+            return self.construct_document(node)
+        return None
+
+    def construct_document(self, node):
+        data = self.construct_object(node)
+        while self.state_generators:
+            state_generators = self.state_generators
+            self.state_generators = []
+            for generator in state_generators:
+                for dummy in generator:
+                    pass
+        self.constructed_objects = {}
+        self.recursive_objects = {}
+        self.deep_construct = False
+        return data
+
+    def construct_object(self, node, deep=False):
+        if node in self.constructed_objects:
+            return self.constructed_objects[node]
+        if deep:
+            old_deep = self.deep_construct
+            self.deep_construct = True
+        if node in self.recursive_objects:
+            raise ConstructorError(None, None,
+                    "found unconstructable recursive node", node.start_mark)
+        self.recursive_objects[node] = None
+        constructor = None
+        tag_suffix = None
+        if node.tag in self.yaml_constructors:
+            constructor = self.yaml_constructors[node.tag]
+        else:
+            for tag_prefix in self.yaml_multi_constructors:
+                if node.tag.startswith(tag_prefix):
+                    tag_suffix = node.tag[len(tag_prefix):]
+                    constructor = self.yaml_multi_constructors[tag_prefix]
+                    break
+            else:
+                if None in self.yaml_multi_constructors:
+                    tag_suffix = node.tag
+                    constructor = self.yaml_multi_constructors[None]
+                elif None in self.yaml_constructors:
+                    constructor = self.yaml_constructors[None]
+                elif isinstance(node, ScalarNode):
+                    constructor = self.__class__.construct_scalar
+                elif isinstance(node, SequenceNode):
+                    constructor = self.__class__.construct_sequence
+                elif isinstance(node, MappingNode):
+                    constructor = self.__class__.construct_mapping
+        if tag_suffix is None:
+            data = constructor(self, node)
+        else:
+            data = constructor(self, tag_suffix, node)
+        if isinstance(data, types.GeneratorType):
+            generator = data
+            data = generator.next()
+            if self.deep_construct:
+                for dummy in generator:
+                    pass
+            else:
+                self.state_generators.append(generator)
+        self.constructed_objects[node] = data
+        del self.recursive_objects[node]
+        if deep:
+            self.deep_construct = old_deep
+        return data
+
+    def construct_scalar(self, node):
+        if not isinstance(node, ScalarNode):
+            raise ConstructorError(None, None,
+                    "expected a scalar node, but found %s" % node.id,
+                    node.start_mark)
+        return node.value
+
+    def construct_sequence(self, node, deep=False):
+        if not isinstance(node, SequenceNode):
+            raise ConstructorError(None, None,
+                    "expected a sequence node, but found %s" % node.id,
+                    node.start_mark)
+        return [self.construct_object(child, deep=deep)
+                for child in node.value]
+
+    def construct_mapping(self, node, deep=False):
+        if not isinstance(node, MappingNode):
+            raise ConstructorError(None, None,
+                    "expected a mapping node, but found %s" % node.id,
+                    node.start_mark)
+        mapping = {}
+        for key_node, value_node in node.value:
+            key = self.construct_object(key_node, deep=deep)
+            try:
+                hash(key)
+            except TypeError, exc:
+                raise ConstructorError("while constructing a mapping", node.start_mark,
+                        "found unacceptable key (%s)" % exc, key_node.start_mark)
+            value = self.construct_object(value_node, deep=deep)
+            if key in mapping:
+                raise ConstructorError("while constructing a mapping", node.start_mark,
+                                       "found already in-use key (%s)" % key, key_node.start_mark)
+            mapping[key] = value
+        return mapping
+
+    def construct_pairs(self, node, deep=False):
+        if not isinstance(node, MappingNode):
+            raise ConstructorError(None, None,
+                    "expected a mapping node, but found %s" % node.id,
+                    node.start_mark)
+        pairs = []
+        for key_node, value_node in node.value:
+            key = self.construct_object(key_node, deep=deep)
+            value = self.construct_object(value_node, deep=deep)
+            pairs.append((key, value))
+        return pairs
+
+    def add_constructor(cls, tag, constructor):
+        if not 'yaml_constructors' in cls.__dict__:
+            cls.yaml_constructors = cls.yaml_constructors.copy()
+        cls.yaml_constructors[tag] = constructor
+    add_constructor = classmethod(add_constructor)
+
+    def add_multi_constructor(cls, tag_prefix, multi_constructor):
+        if not 'yaml_multi_constructors' in cls.__dict__:
+            cls.yaml_multi_constructors = cls.yaml_multi_constructors.copy()
+        cls.yaml_multi_constructors[tag_prefix] = multi_constructor
+    add_multi_constructor = classmethod(add_multi_constructor)
+
+class SafeConstructor(BaseConstructor):
+
+    def construct_scalar(self, node):
+        if isinstance(node, MappingNode):
+            for key_node, value_node in node.value:
+                if key_node.tag == u'tag:yaml.org,2002:value':
+                    return self.construct_scalar(value_node)
+        return BaseConstructor.construct_scalar(self, node)
+
+    def flatten_mapping(self, node):
+        merge = []
+        index = 0
+        while index < len(node.value):
+            key_node, value_node = node.value[index]
+            if key_node.tag == u'tag:yaml.org,2002:merge':
+                del node.value[index]
+                if isinstance(value_node, MappingNode):
+                    self.flatten_mapping(value_node)
+                    merge.extend(value_node.value)
+                elif isinstance(value_node, SequenceNode):
+                    submerge = []
+                    for subnode in value_node.value:
+                        if not isinstance(subnode, MappingNode):
+                            raise ConstructorError("while constructing a mapping",
+                                    node.start_mark,
+                                    "expected a mapping for merging, but found %s"
+                                    % subnode.id, subnode.start_mark)
+                        self.flatten_mapping(subnode)
+                        submerge.append(subnode.value)
+                    submerge.reverse()
+                    for value in submerge:
+                        merge.extend(value)
+                else:
+                    raise ConstructorError("while constructing a mapping", node.start_mark,
+                            "expected a mapping or list of mappings for merging, but found %s"
+                            % value_node.id, value_node.start_mark)
+            elif key_node.tag == u'tag:yaml.org,2002:value':
+                key_node.tag = u'tag:yaml.org,2002:str'
+                index += 1
+            else:
+                index += 1
+        if merge:
+            node.value = merge + node.value
+
+    def construct_mapping(self, node, deep=False):
+        if isinstance(node, MappingNode):
+            self.flatten_mapping(node)
+        return BaseConstructor.construct_mapping(self, node, deep=deep)
+
+    def construct_yaml_null(self, node):
+        self.construct_scalar(node)
+        return None
+
+    bool_values = {
+        u'yes':     True,
+        u'no':      False,
+        u'true':    True,
+        u'false':   False,
+        u'on':      True,
+        u'off':     False,
+    }
+
+    def construct_yaml_bool(self, node):
+        value = self.construct_scalar(node)
+        return self.bool_values[value.lower()]
+
+    def construct_yaml_int(self, node):
+        value = str(self.construct_scalar(node))
+        value = value.replace('_', '')
+        sign = +1
+        if value[0] == '-':
+            sign = -1
+        if value[0] in '+-':
+            value = value[1:]
+        if value == '0':
+            return 0
+        elif value.startswith('0b'):
+            return sign*int(value[2:], 2)
+        elif value.startswith('0x'):
+            return sign*int(value[2:], 16)
+        elif value[0] == '0':
+            return sign*int(value, 8)
+        elif ':' in value:
+            digits = [int(part) for part in value.split(':')]
+            digits.reverse()
+            base = 1
+            value = 0
+            for digit in digits:
+                value += digit*base
+                base *= 60
+            return sign*value
+        else:
+            return sign*int(value)
+
+    inf_value = 1e300
+    while inf_value != inf_value*inf_value:
+        inf_value *= inf_value
+    nan_value = -inf_value/inf_value   # Trying to make a quiet NaN (like C99).
+
+    def construct_yaml_float(self, node):
+        value = str(self.construct_scalar(node))
+        value = value.replace('_', '').lower()
+        sign = +1
+        if value[0] == '-':
+            sign = -1
+        if value[0] in '+-':
+            value = value[1:]
+        if value == '.inf':
+            return sign*self.inf_value
+        elif value == '.nan':
+            return self.nan_value
+        elif ':' in value:
+            digits = [float(part) for part in value.split(':')]
+            digits.reverse()
+            base = 1
+            value = 0.0
+            for digit in digits:
+                value += digit*base
+                base *= 60
+            return sign*value
+        else:
+            return sign*float(value)
+
+    def construct_yaml_binary(self, node):
+        value = self.construct_scalar(node)
+        try:
+            return str(value).decode('base64')
+        except (binascii.Error, UnicodeEncodeError), exc:
+            raise ConstructorError(None, None,
+                    "failed to decode base64 data: %s" % exc, node.start_mark) 
+
+    timestamp_regexp = re.compile(
+            ur'''^(?P<year>[0-9][0-9][0-9][0-9])
+                -(?P<month>[0-9][0-9]?)
+                -(?P<day>[0-9][0-9]?)
+                (?:(?:[Tt]|[ \t]+)
+                (?P<hour>[0-9][0-9]?)
+                :(?P<minute>[0-9][0-9])
+                :(?P<second>[0-9][0-9])
+                (?:\.(?P<fraction>[0-9]*))?
+                (?:[ \t]*(?P<tz>Z|(?P<tz_sign>[-+])(?P<tz_hour>[0-9][0-9]?)
+                (?::(?P<tz_minute>[0-9][0-9]))?))?)?$''', re.X)
+
+    def construct_yaml_timestamp(self, node):
+        value = self.construct_scalar(node)
+        match = self.timestamp_regexp.match(node.value)
+        values = match.groupdict()
+        year = int(values['year'])
+        month = int(values['month'])
+        day = int(values['day'])
+        if not values['hour']:
+            return datetime.date(year, month, day)
+        hour = int(values['hour'])
+        minute = int(values['minute'])
+        second = int(values['second'])
+        fraction = 0
+        if values['fraction']:
+            fraction = values['fraction'][:6]
+            while len(fraction) < 6:
+                fraction += '0'
+            fraction = int(fraction)
+        delta = None
+        if values['tz_sign']:
+            tz_hour = int(values['tz_hour'])
+            tz_minute = int(values['tz_minute'] or 0)
+            delta = datetime.timedelta(hours=tz_hour, minutes=tz_minute)
+            if values['tz_sign'] == '-':
+                delta = -delta
+        data = datetime.datetime(year, month, day, hour, minute, second, fraction)
+        if delta:
+            data -= delta
+        return data
+
+    def construct_yaml_omap(self, node):
+        # Note: we do not check for duplicate keys, because it's too
+        # CPU-expensive.
+        omap = []
+        yield omap
+        if not isinstance(node, SequenceNode):
+            raise ConstructorError("while constructing an ordered map", node.start_mark,
+                    "expected a sequence, but found %s" % node.id, node.start_mark)
+        for subnode in node.value:
+            if not isinstance(subnode, MappingNode):
+                raise ConstructorError("while constructing an ordered map", node.start_mark,
+                        "expected a mapping of length 1, but found %s" % subnode.id,
+                        subnode.start_mark)
+            if len(subnode.value) != 1:
+                raise ConstructorError("while constructing an ordered map", node.start_mark,
+                        "expected a single mapping item, but found %d items" % len(subnode.value),
+                        subnode.start_mark)
+            key_node, value_node = subnode.value[0]
+            key = self.construct_object(key_node)
+            value = self.construct_object(value_node)
+            omap.append((key, value))
+
+    def construct_yaml_pairs(self, node):
+        # Note: the same code as `construct_yaml_omap`.
+        pairs = []
+        yield pairs
+        if not isinstance(node, SequenceNode):
+            raise ConstructorError("while constructing pairs", node.start_mark,
+                    "expected a sequence, but found %s" % node.id, node.start_mark)
+        for subnode in node.value:
+            if not isinstance(subnode, MappingNode):
+                raise ConstructorError("while constructing pairs", node.start_mark,
+                        "expected a mapping of length 1, but found %s" % subnode.id,
+                        subnode.start_mark)
+            if len(subnode.value) != 1:
+                raise ConstructorError("while constructing pairs", node.start_mark,
+                        "expected a single mapping item, but found %d items" % len(subnode.value),
+                        subnode.start_mark)
+            key_node, value_node = subnode.value[0]
+            key = self.construct_object(key_node)
+            value = self.construct_object(value_node)
+            pairs.append((key, value))
+
+    def construct_yaml_set(self, node):
+        data = set()
+        yield data
+        value = self.construct_mapping(node)
+        data.update(value)
+
+    def construct_yaml_str(self, node):
+        value = self.construct_scalar(node)
+        try:
+            return value.encode('ascii')
+        except UnicodeEncodeError:
+            return value
+
+    def construct_yaml_seq(self, node):
+        data = []
+        yield data
+        data.extend(self.construct_sequence(node))
+
+    def construct_yaml_map(self, node):
+        data = {}
+        yield data
+        value = self.construct_mapping(node)
+        data.update(value)
+
+    def construct_yaml_object(self, node, cls):
+        data = cls.__new__(cls)
+        yield data
+        if hasattr(data, '__setstate__'):
+            state = self.construct_mapping(node, deep=True)
+            data.__setstate__(state)
+        else:
+            state = self.construct_mapping(node)
+            data.__dict__.update(state)
+
+    def construct_undefined(self, node):
+        raise ConstructorError(None, None,
+                "could not determine a constructor for the tag %r" % node.tag.encode('utf-8'),
+                node.start_mark)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:null',
+        SafeConstructor.construct_yaml_null)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:bool',
+        SafeConstructor.construct_yaml_bool)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:int',
+        SafeConstructor.construct_yaml_int)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:float',
+        SafeConstructor.construct_yaml_float)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:binary',
+        SafeConstructor.construct_yaml_binary)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:timestamp',
+        SafeConstructor.construct_yaml_timestamp)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:omap',
+        SafeConstructor.construct_yaml_omap)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:pairs',
+        SafeConstructor.construct_yaml_pairs)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:set',
+        SafeConstructor.construct_yaml_set)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:str',
+        SafeConstructor.construct_yaml_str)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:seq',
+        SafeConstructor.construct_yaml_seq)
+
+SafeConstructor.add_constructor(
+        u'tag:yaml.org,2002:map',
+        SafeConstructor.construct_yaml_map)
+
+SafeConstructor.add_constructor(None,
+        SafeConstructor.construct_undefined)
+
+class Constructor(SafeConstructor):
+
+    def construct_python_str(self, node):
+        return self.construct_scalar(node).encode('utf-8')
+
+    def construct_python_unicode(self, node):
+        return self.construct_scalar(node)
+
+    def construct_python_long(self, node):
+        return long(self.construct_yaml_int(node))
+
+    def construct_python_complex(self, node):
+       return complex(self.construct_scalar(node))
+
+    def construct_python_tuple(self, node):
+        return tuple(self.construct_sequence(node))
+
+    def find_python_module(self, name, mark):
+        if not name:
+            raise ConstructorError("while constructing a Python module", mark,
+                    "expected non-empty name appended to the tag", mark)
+        try:
+            __import__(name)
+        except ImportError, exc:
+            raise ConstructorError("while constructing a Python module", mark,
+                    "cannot find module %r (%s)" % (name.encode('utf-8'), exc), mark)
+        return sys.modules[name]
+
+    def find_python_name(self, name, mark):
+        if not name:
+            raise ConstructorError("while constructing a Python object", mark,
+                    "expected non-empty name appended to the tag", mark)
+        if u'.' in name:
+            module_name, object_name = name.rsplit('.', 1)
+        else:
+            module_name = '__builtin__'
+            object_name = name
+        try:
+            __import__(module_name)
+        except ImportError, exc:
+            raise ConstructorError("while constructing a Python object", mark,
+                    "cannot find module %r (%s)" % (module_name.encode('utf-8'), exc), mark)
+        module = sys.modules[module_name]
+        if not hasattr(module, object_name):
+            raise ConstructorError("while constructing a Python object", mark,
+                    "cannot find %r in the module %r" % (object_name.encode('utf-8'),
+                        module.__name__), mark)
+        return getattr(module, object_name)
+
+    def construct_python_name(self, suffix, node):
+        value = self.construct_scalar(node)
+        if value:
+            raise ConstructorError("while constructing a Python name", node.start_mark,
+                    "expected the empty value, but found %r" % value.encode('utf-8'),
+                    node.start_mark)
+        return self.find_python_name(suffix, node.start_mark)
+
+    def construct_python_module(self, suffix, node):
+        value = self.construct_scalar(node)
+        if value:
+            raise ConstructorError("while constructing a Python module", node.start_mark,
+                    "expected the empty value, but found %r" % value.encode('utf-8'),
+                    node.start_mark)
+        return self.find_python_module(suffix, node.start_mark)
+
+    class classobj: pass
+
+    def make_python_instance(self, suffix, node,
+            args=None, kwds=None, newobj=False):
+        if not args:
+            args = []
+        if not kwds:
+            kwds = {}
+        cls = self.find_python_name(suffix, node.start_mark)
+        if newobj and isinstance(cls, type(self.classobj))  \
+                and not args and not kwds:
+            instance = self.classobj()
+            instance.__class__ = cls
+            return instance
+        elif newobj and isinstance(cls, type):
+            return cls.__new__(cls, *args, **kwds)
+        else:
+            return cls(*args, **kwds)
+
+    def set_python_instance_state(self, instance, state):
+        if hasattr(instance, '__setstate__'):
+            instance.__setstate__(state)
+        else:
+            slotstate = {}
+            if isinstance(state, tuple) and len(state) == 2:
+                state, slotstate = state
+            if hasattr(instance, '__dict__'):
+                instance.__dict__.update(state)
+            elif state:
+                slotstate.update(state)
+            for key, value in slotstate.items():
+                setattr(object, key, value)
+
+    def construct_python_object(self, suffix, node):
+        # Format:
+        #   !!python/object:module.name { ... state ... }
+        instance = self.make_python_instance(suffix, node, newobj=True)
+        yield instance
+        deep = hasattr(instance, '__setstate__')
+        state = self.construct_mapping(node, deep=deep)
+        self.set_python_instance_state(instance, state)
+
+    def construct_python_object_apply(self, suffix, node, newobj=False):
+        # Format:
+        #   !!python/object/apply       # (or !!python/object/new)
+        #   args: [ ... arguments ... ]
+        #   kwds: { ... keywords ... }
+        #   state: ... state ...
+        #   listitems: [ ... listitems ... ]
+        #   dictitems: { ... dictitems ... }
+        # or short format:
+        #   !!python/object/apply [ ... arguments ... ]
+        # The difference between !!python/object/apply and !!python/object/new
+        # is how an object is created, check make_python_instance for details.
+        if isinstance(node, SequenceNode):
+            args = self.construct_sequence(node, deep=True)
+            kwds = {}
+            state = {}
+            listitems = []
+            dictitems = {}
+        else:
+            value = self.construct_mapping(node, deep=True)
+            args = value.get('args', [])
+            kwds = value.get('kwds', {})
+            state = value.get('state', {})
+            listitems = value.get('listitems', [])
+            dictitems = value.get('dictitems', {})
+        instance = self.make_python_instance(suffix, node, args, kwds, newobj)
+        if state:
+            self.set_python_instance_state(instance, state)
+        if listitems:
+            instance.extend(listitems)
+        if dictitems:
+            for key in dictitems:
+                instance[key] = dictitems[key]
+        return instance
+
+    def construct_python_object_new(self, suffix, node):
+        return self.construct_python_object_apply(suffix, node, newobj=True)
+
+Constructor.add_constructor(
+    u'tag:yaml.org,2002:python/none',
+    Constructor.construct_yaml_null)
+
+Constructor.add_constructor(
+    u'tag:yaml.org,2002:python/bool',
+    Constructor.construct_yaml_bool)
+
+Constructor.add_constructor(
+    u'tag:yaml.org,2002:python/str',
+    Constructor.construct_python_str)
+
+Constructor.add_constructor(
+    u'tag:yaml.org,2002:python/unicode',
+    Constructor.construct_python_unicode)
+
+Constructor.add_constructor(
+    u'tag:yaml.org,2002:python/int',
+    Constructor.construct_yaml_int)
+
+Constructor.add_constructor(
+    u'tag:yaml.org,2002:python/long',
+    Constructor.construct_python_long)
+
+Constructor.add_constructor(
+    u'tag:yaml.org,2002:python/float',
+    Constructor.construct_yaml_float)
+
+Constructor.add_constructor(
+    u'tag:yaml.org,2002:python/complex',
+    Constructor.construct_python_complex)
+
+Constructor.add_constructor(
+    u'tag:yaml.org,2002:python/list',
+    Constructor.construct_yaml_seq)
+
+Constructor.add_constructor(
+    u'tag:yaml.org,2002:python/tuple',
+    Constructor.construct_python_tuple)
+
+Constructor.add_constructor(
+    u'tag:yaml.org,2002:python/dict',
+    Constructor.construct_yaml_map)
+
+Constructor.add_multi_constructor(
+    u'tag:yaml.org,2002:python/name:',
+    Constructor.construct_python_name)
+
+Constructor.add_multi_constructor(
+    u'tag:yaml.org,2002:python/module:',
+    Constructor.construct_python_module)
+
+Constructor.add_multi_constructor(
+    u'tag:yaml.org,2002:python/object:',
+    Constructor.construct_python_object)
+
+Constructor.add_multi_constructor(
+    u'tag:yaml.org,2002:python/object/apply:',
+    Constructor.construct_python_object_apply)
+
+Constructor.add_multi_constructor(
+    u'tag:yaml.org,2002:python/object/new:',
+    Constructor.construct_python_object_new)
+
diff --git a/lib/spack/external/yaml/dumper.py b/lib/spack/external/yaml/dumper.py
new file mode 100644
index 0000000000..f811d2c919
--- /dev/null
+++ b/lib/spack/external/yaml/dumper.py
@@ -0,0 +1,62 @@
+
+__all__ = ['BaseDumper', 'SafeDumper', 'Dumper']
+
+from emitter import *
+from serializer import *
+from representer import *
+from resolver import *
+
+class BaseDumper(Emitter, Serializer, BaseRepresenter, BaseResolver):
+
+    def __init__(self, stream,
+            default_style=None, default_flow_style=None,
+            canonical=None, indent=None, width=None,
+            allow_unicode=None, line_break=None,
+            encoding=None, explicit_start=None, explicit_end=None,
+            version=None, tags=None):
+        Emitter.__init__(self, stream, canonical=canonical,
+                indent=indent, width=width,
+                allow_unicode=allow_unicode, line_break=line_break)
+        Serializer.__init__(self, encoding=encoding,
+                explicit_start=explicit_start, explicit_end=explicit_end,
+                version=version, tags=tags)
+        Representer.__init__(self, default_style=default_style,
+                default_flow_style=default_flow_style)
+        Resolver.__init__(self)
+
+class SafeDumper(Emitter, Serializer, SafeRepresenter, Resolver):
+
+    def __init__(self, stream,
+            default_style=None, default_flow_style=None,
+            canonical=None, indent=None, width=None,
+            allow_unicode=None, line_break=None,
+            encoding=None, explicit_start=None, explicit_end=None,
+            version=None, tags=None):
+        Emitter.__init__(self, stream, canonical=canonical,
+                indent=indent, width=width,
+                allow_unicode=allow_unicode, line_break=line_break)
+        Serializer.__init__(self, encoding=encoding,
+                explicit_start=explicit_start, explicit_end=explicit_end,
+                version=version, tags=tags)
+        SafeRepresenter.__init__(self, default_style=default_style,
+                default_flow_style=default_flow_style)
+        Resolver.__init__(self)
+
+class Dumper(Emitter, Serializer, Representer, Resolver):
+
+    def __init__(self, stream,
+            default_style=None, default_flow_style=None,
+            canonical=None, indent=None, width=None,
+            allow_unicode=None, line_break=None,
+            encoding=None, explicit_start=None, explicit_end=None,
+            version=None, tags=None):
+        Emitter.__init__(self, stream, canonical=canonical,
+                indent=indent, width=width,
+                allow_unicode=allow_unicode, line_break=line_break)
+        Serializer.__init__(self, encoding=encoding,
+                explicit_start=explicit_start, explicit_end=explicit_end,
+                version=version, tags=tags)
+        Representer.__init__(self, default_style=default_style,
+                default_flow_style=default_flow_style)
+        Resolver.__init__(self)
+
diff --git a/lib/spack/external/yaml/emitter.py b/lib/spack/external/yaml/emitter.py
new file mode 100644
index 0000000000..e5bcdcccbb
--- /dev/null
+++ b/lib/spack/external/yaml/emitter.py
@@ -0,0 +1,1140 @@
+
+# Emitter expects events obeying the following grammar:
+# stream ::= STREAM-START document* STREAM-END
+# document ::= DOCUMENT-START node DOCUMENT-END
+# node ::= SCALAR | sequence | mapping
+# sequence ::= SEQUENCE-START node* SEQUENCE-END
+# mapping ::= MAPPING-START (node node)* MAPPING-END
+
+__all__ = ['Emitter', 'EmitterError']
+
+from error import YAMLError
+from events import *
+
+class EmitterError(YAMLError):
+    pass
+
+class ScalarAnalysis(object):
+    def __init__(self, scalar, empty, multiline,
+            allow_flow_plain, allow_block_plain,
+            allow_single_quoted, allow_double_quoted,
+            allow_block):
+        self.scalar = scalar
+        self.empty = empty
+        self.multiline = multiline
+        self.allow_flow_plain = allow_flow_plain
+        self.allow_block_plain = allow_block_plain
+        self.allow_single_quoted = allow_single_quoted
+        self.allow_double_quoted = allow_double_quoted
+        self.allow_block = allow_block
+
+class Emitter(object):
+
+    DEFAULT_TAG_PREFIXES = {
+        u'!' : u'!',
+        u'tag:yaml.org,2002:' : u'!!',
+    }
+
+    def __init__(self, stream, canonical=None, indent=None, width=None,
+            allow_unicode=None, line_break=None):
+
+        # The stream should have the methods `write` and possibly `flush`.
+        self.stream = stream
+
+        # Encoding can be overriden by STREAM-START.
+        self.encoding = None
+
+        # Emitter is a state machine with a stack of states to handle nested
+        # structures.
+        self.states = []
+        self.state = self.expect_stream_start
+
+        # Current event and the event queue.
+        self.events = []
+        self.event = None
+
+        # The current indentation level and the stack of previous indents.
+        self.indents = []
+        self.indent = None
+
+        # Flow level.
+        self.flow_level = 0
+
+        # Contexts.
+        self.root_context = False
+        self.sequence_context = False
+        self.mapping_context = False
+        self.simple_key_context = False
+
+        # Characteristics of the last emitted character:
+        #  - current position.
+        #  - is it a whitespace?
+        #  - is it an indention character
+        #    (indentation space, '-', '?', or ':')?
+        self.line = 0
+        self.column = 0
+        self.whitespace = True
+        self.indention = True
+
+        # Whether the document requires an explicit document indicator
+        self.open_ended = False
+
+        # Formatting details.
+        self.canonical = canonical
+        self.allow_unicode = allow_unicode
+        self.best_indent = 2
+        if indent and 1 < indent < 10:
+            self.best_indent = indent
+        self.best_width = 80
+        if width and width > self.best_indent*2:
+            self.best_width = width
+        self.best_line_break = u'\n'
+        if line_break in [u'\r', u'\n', u'\r\n']:
+            self.best_line_break = line_break
+
+        # Tag prefixes.
+        self.tag_prefixes = None
+
+        # Prepared anchor and tag.
+        self.prepared_anchor = None
+        self.prepared_tag = None
+
+        # Scalar analysis and style.
+        self.analysis = None
+        self.style = None
+
+    def dispose(self):
+        # Reset the state attributes (to clear self-references)
+        self.states = []
+        self.state = None
+
+    def emit(self, event):
+        self.events.append(event)
+        while not self.need_more_events():
+            self.event = self.events.pop(0)
+            self.state()
+            self.event = None
+
+    # In some cases, we wait for a few next events before emitting.
+
+    def need_more_events(self):
+        if not self.events:
+            return True
+        event = self.events[0]
+        if isinstance(event, DocumentStartEvent):
+            return self.need_events(1)
+        elif isinstance(event, SequenceStartEvent):
+            return self.need_events(2)
+        elif isinstance(event, MappingStartEvent):
+            return self.need_events(3)
+        else:
+            return False
+
+    def need_events(self, count):
+        level = 0
+        for event in self.events[1:]:
+            if isinstance(event, (DocumentStartEvent, CollectionStartEvent)):
+                level += 1
+            elif isinstance(event, (DocumentEndEvent, CollectionEndEvent)):
+                level -= 1
+            elif isinstance(event, StreamEndEvent):
+                level = -1
+            if level < 0:
+                return False
+        return (len(self.events) < count+1)
+
+    def increase_indent(self, flow=False, indentless=False):
+        self.indents.append(self.indent)
+        if self.indent is None:
+            if flow:
+                self.indent = self.best_indent
+            else:
+                self.indent = 0
+        elif not indentless:
+            self.indent += self.best_indent
+
+    # States.
+
+    # Stream handlers.
+
+    def expect_stream_start(self):
+        if isinstance(self.event, StreamStartEvent):
+            if self.event.encoding and not getattr(self.stream, 'encoding', None):
+                self.encoding = self.event.encoding
+            self.write_stream_start()
+            self.state = self.expect_first_document_start
+        else:
+            raise EmitterError("expected StreamStartEvent, but got %s"
+                    % self.event)
+
+    def expect_nothing(self):
+        raise EmitterError("expected nothing, but got %s" % self.event)
+
+    # Document handlers.
+
+    def expect_first_document_start(self):
+        return self.expect_document_start(first=True)
+
+    def expect_document_start(self, first=False):
+        if isinstance(self.event, DocumentStartEvent):
+            if (self.event.version or self.event.tags) and self.open_ended:
+                self.write_indicator(u'...', True)
+                self.write_indent()
+            if self.event.version:
+                version_text = self.prepare_version(self.event.version)
+                self.write_version_directive(version_text)
+            self.tag_prefixes = self.DEFAULT_TAG_PREFIXES.copy()
+            if self.event.tags:
+                handles = self.event.tags.keys()
+                handles.sort()
+                for handle in handles:
+                    prefix = self.event.tags[handle]
+                    self.tag_prefixes[prefix] = handle
+                    handle_text = self.prepare_tag_handle(handle)
+                    prefix_text = self.prepare_tag_prefix(prefix)
+                    self.write_tag_directive(handle_text, prefix_text)
+            implicit = (first and not self.event.explicit and not self.canonical
+                    and not self.event.version and not self.event.tags
+                    and not self.check_empty_document())
+            if not implicit:
+                self.write_indent()
+                self.write_indicator(u'---', True)
+                if self.canonical:
+                    self.write_indent()
+            self.state = self.expect_document_root
+        elif isinstance(self.event, StreamEndEvent):
+            if self.open_ended:
+                self.write_indicator(u'...', True)
+                self.write_indent()
+            self.write_stream_end()
+            self.state = self.expect_nothing
+        else:
+            raise EmitterError("expected DocumentStartEvent, but got %s"
+                    % self.event)
+
+    def expect_document_end(self):
+        if isinstance(self.event, DocumentEndEvent):
+            self.write_indent()
+            if self.event.explicit:
+                self.write_indicator(u'...', True)
+                self.write_indent()
+            self.flush_stream()
+            self.state = self.expect_document_start
+        else:
+            raise EmitterError("expected DocumentEndEvent, but got %s"
+                    % self.event)
+
+    def expect_document_root(self):
+        self.states.append(self.expect_document_end)
+        self.expect_node(root=True)
+
+    # Node handlers.
+
+    def expect_node(self, root=False, sequence=False, mapping=False,
+            simple_key=False):
+        self.root_context = root
+        self.sequence_context = sequence
+        self.mapping_context = mapping
+        self.simple_key_context = simple_key
+        if isinstance(self.event, AliasEvent):
+            self.expect_alias()
+        elif isinstance(self.event, (ScalarEvent, CollectionStartEvent)):
+            self.process_anchor(u'&')
+            self.process_tag()
+            if isinstance(self.event, ScalarEvent):
+                self.expect_scalar()
+            elif isinstance(self.event, SequenceStartEvent):
+                if self.flow_level or self.canonical or self.event.flow_style   \
+                        or self.check_empty_sequence():
+                    self.expect_flow_sequence()
+                else:
+                    self.expect_block_sequence()
+            elif isinstance(self.event, MappingStartEvent):
+                if self.flow_level or self.canonical or self.event.flow_style   \
+                        or self.check_empty_mapping():
+                    self.expect_flow_mapping()
+                else:
+                    self.expect_block_mapping()
+        else:
+            raise EmitterError("expected NodeEvent, but got %s" % self.event)
+
+    def expect_alias(self):
+        if self.event.anchor is None:
+            raise EmitterError("anchor is not specified for alias")
+        self.process_anchor(u'*')
+        self.state = self.states.pop()
+
+    def expect_scalar(self):
+        self.increase_indent(flow=True)
+        self.process_scalar()
+        self.indent = self.indents.pop()
+        self.state = self.states.pop()
+
+    # Flow sequence handlers.
+
+    def expect_flow_sequence(self):
+        self.write_indicator(u'[', True, whitespace=True)
+        self.flow_level += 1
+        self.increase_indent(flow=True)
+        self.state = self.expect_first_flow_sequence_item
+
+    def expect_first_flow_sequence_item(self):
+        if isinstance(self.event, SequenceEndEvent):
+            self.indent = self.indents.pop()
+            self.flow_level -= 1
+            self.write_indicator(u']', False)
+            self.state = self.states.pop()
+        else:
+            if self.canonical or self.column > self.best_width:
+                self.write_indent()
+            self.states.append(self.expect_flow_sequence_item)
+            self.expect_node(sequence=True)
+
+    def expect_flow_sequence_item(self):
+        if isinstance(self.event, SequenceEndEvent):
+            self.indent = self.indents.pop()
+            self.flow_level -= 1
+            if self.canonical:
+                self.write_indicator(u',', False)
+                self.write_indent()
+            self.write_indicator(u']', False)
+            self.state = self.states.pop()
+        else:
+            self.write_indicator(u',', False)
+            if self.canonical or self.column > self.best_width:
+                self.write_indent()
+            self.states.append(self.expect_flow_sequence_item)
+            self.expect_node(sequence=True)
+
+    # Flow mapping handlers.
+
+    def expect_flow_mapping(self):
+        self.write_indicator(u'{', True, whitespace=True)
+        self.flow_level += 1
+        self.increase_indent(flow=True)
+        self.state = self.expect_first_flow_mapping_key
+
+    def expect_first_flow_mapping_key(self):
+        if isinstance(self.event, MappingEndEvent):
+            self.indent = self.indents.pop()
+            self.flow_level -= 1
+            self.write_indicator(u'}', False)
+            self.state = self.states.pop()
+        else:
+            if self.canonical or self.column > self.best_width:
+                self.write_indent()
+            if not self.canonical and self.check_simple_key():
+                self.states.append(self.expect_flow_mapping_simple_value)
+                self.expect_node(mapping=True, simple_key=True)
+            else:
+                self.write_indicator(u'?', True)
+                self.states.append(self.expect_flow_mapping_value)
+                self.expect_node(mapping=True)
+
+    def expect_flow_mapping_key(self):
+        if isinstance(self.event, MappingEndEvent):
+            self.indent = self.indents.pop()
+            self.flow_level -= 1
+            if self.canonical:
+                self.write_indicator(u',', False)
+                self.write_indent()
+            self.write_indicator(u'}', False)
+            self.state = self.states.pop()
+        else:
+            self.write_indicator(u',', False)
+            if self.canonical or self.column > self.best_width:
+                self.write_indent()
+            if not self.canonical and self.check_simple_key():
+                self.states.append(self.expect_flow_mapping_simple_value)
+                self.expect_node(mapping=True, simple_key=True)
+            else:
+                self.write_indicator(u'?', True)
+                self.states.append(self.expect_flow_mapping_value)
+                self.expect_node(mapping=True)
+
+    def expect_flow_mapping_simple_value(self):
+        self.write_indicator(u':', False)
+        self.states.append(self.expect_flow_mapping_key)
+        self.expect_node(mapping=True)
+
+    def expect_flow_mapping_value(self):
+        if self.canonical or self.column > self.best_width:
+            self.write_indent()
+        self.write_indicator(u':', True)
+        self.states.append(self.expect_flow_mapping_key)
+        self.expect_node(mapping=True)
+
+    # Block sequence handlers.
+
+    def expect_block_sequence(self):
+        indentless = (self.mapping_context and not self.indention)
+        self.increase_indent(flow=False, indentless=indentless)
+        self.state = self.expect_first_block_sequence_item
+
+    def expect_first_block_sequence_item(self):
+        return self.expect_block_sequence_item(first=True)
+
+    def expect_block_sequence_item(self, first=False):
+        if not first and isinstance(self.event, SequenceEndEvent):
+            self.indent = self.indents.pop()
+            self.state = self.states.pop()
+        else:
+            self.write_indent()
+            self.write_indicator(u'-', True, indention=True)
+            self.states.append(self.expect_block_sequence_item)
+            self.expect_node(sequence=True)
+
+    # Block mapping handlers.
+
+    def expect_block_mapping(self):
+        self.increase_indent(flow=False)
+        self.state = self.expect_first_block_mapping_key
+
+    def expect_first_block_mapping_key(self):
+        return self.expect_block_mapping_key(first=True)
+
+    def expect_block_mapping_key(self, first=False):
+        if not first and isinstance(self.event, MappingEndEvent):
+            self.indent = self.indents.pop()
+            self.state = self.states.pop()
+        else:
+            self.write_indent()
+            if self.check_simple_key():
+                self.states.append(self.expect_block_mapping_simple_value)
+                self.expect_node(mapping=True, simple_key=True)
+            else:
+                self.write_indicator(u'?', True, indention=True)
+                self.states.append(self.expect_block_mapping_value)
+                self.expect_node(mapping=True)
+
+    def expect_block_mapping_simple_value(self):
+        self.write_indicator(u':', False)
+        self.states.append(self.expect_block_mapping_key)
+        self.expect_node(mapping=True)
+
+    def expect_block_mapping_value(self):
+        self.write_indent()
+        self.write_indicator(u':', True, indention=True)
+        self.states.append(self.expect_block_mapping_key)
+        self.expect_node(mapping=True)
+
+    # Checkers.
+
+    def check_empty_sequence(self):
+        return (isinstance(self.event, SequenceStartEvent) and self.events
+                and isinstance(self.events[0], SequenceEndEvent))
+
+    def check_empty_mapping(self):
+        return (isinstance(self.event, MappingStartEvent) and self.events
+                and isinstance(self.events[0], MappingEndEvent))
+
+    def check_empty_document(self):
+        if not isinstance(self.event, DocumentStartEvent) or not self.events:
+            return False
+        event = self.events[0]
+        return (isinstance(event, ScalarEvent) and event.anchor is None
+                and event.tag is None and event.implicit and event.value == u'')
+
+    def check_simple_key(self):
+        length = 0
+        if isinstance(self.event, NodeEvent) and self.event.anchor is not None:
+            if self.prepared_anchor is None:
+                self.prepared_anchor = self.prepare_anchor(self.event.anchor)
+            length += len(self.prepared_anchor)
+        if isinstance(self.event, (ScalarEvent, CollectionStartEvent))  \
+                and self.event.tag is not None:
+            if self.prepared_tag is None:
+                self.prepared_tag = self.prepare_tag(self.event.tag)
+            length += len(self.prepared_tag)
+        if isinstance(self.event, ScalarEvent):
+            if self.analysis is None:
+                self.analysis = self.analyze_scalar(self.event.value)
+            length += len(self.analysis.scalar)
+        return (length < 128 and (isinstance(self.event, AliasEvent)
+            or (isinstance(self.event, ScalarEvent)
+                    and not self.analysis.empty and not self.analysis.multiline)
+            or self.check_empty_sequence() or self.check_empty_mapping()))
+
+    # Anchor, Tag, and Scalar processors.
+
+    def process_anchor(self, indicator):
+        if self.event.anchor is None:
+            self.prepared_anchor = None
+            return
+        if self.prepared_anchor is None:
+            self.prepared_anchor = self.prepare_anchor(self.event.anchor)
+        if self.prepared_anchor:
+            self.write_indicator(indicator+self.prepared_anchor, True)
+        self.prepared_anchor = None
+
+    def process_tag(self):
+        tag = self.event.tag
+        if isinstance(self.event, ScalarEvent):
+            if self.style is None:
+                self.style = self.choose_scalar_style()
+            if ((not self.canonical or tag is None) and
+                ((self.style == '' and self.event.implicit[0])
+                        or (self.style != '' and self.event.implicit[1]))):
+                self.prepared_tag = None
+                return
+            if self.event.implicit[0] and tag is None:
+                tag = u'!'
+                self.prepared_tag = None
+        else:
+            if (not self.canonical or tag is None) and self.event.implicit:
+                self.prepared_tag = None
+                return
+        if tag is None:
+            raise EmitterError("tag is not specified")
+        if self.prepared_tag is None:
+            self.prepared_tag = self.prepare_tag(tag)
+        if self.prepared_tag:
+            self.write_indicator(self.prepared_tag, True)
+        self.prepared_tag = None
+
+    def choose_scalar_style(self):
+        if self.analysis is None:
+            self.analysis = self.analyze_scalar(self.event.value)
+        if self.event.style == '"' or self.canonical:
+            return '"'
+        if not self.event.style and self.event.implicit[0]:
+            if (not (self.simple_key_context and
+                    (self.analysis.empty or self.analysis.multiline))
+                and (self.flow_level and self.analysis.allow_flow_plain
+                    or (not self.flow_level and self.analysis.allow_block_plain))):
+                return ''
+        if self.event.style and self.event.style in '|>':
+            if (not self.flow_level and not self.simple_key_context
+                    and self.analysis.allow_block):
+                return self.event.style
+        if not self.event.style or self.event.style == '\'':
+            if (self.analysis.allow_single_quoted and
+                    not (self.simple_key_context and self.analysis.multiline)):
+                return '\''
+        return '"'
+
+    def process_scalar(self):
+        if self.analysis is None:
+            self.analysis = self.analyze_scalar(self.event.value)
+        if self.style is None:
+            self.style = self.choose_scalar_style()
+        split = (not self.simple_key_context)
+        #if self.analysis.multiline and split    \
+        #        and (not self.style or self.style in '\'\"'):
+        #    self.write_indent()
+        if self.style == '"':
+            self.write_double_quoted(self.analysis.scalar, split)
+        elif self.style == '\'':
+            self.write_single_quoted(self.analysis.scalar, split)
+        elif self.style == '>':
+            self.write_folded(self.analysis.scalar)
+        elif self.style == '|':
+            self.write_literal(self.analysis.scalar)
+        else:
+            self.write_plain(self.analysis.scalar, split)
+        self.analysis = None
+        self.style = None
+
+    # Analyzers.
+
+    def prepare_version(self, version):
+        major, minor = version
+        if major != 1:
+            raise EmitterError("unsupported YAML version: %d.%d" % (major, minor))
+        return u'%d.%d' % (major, minor)
+
+    def prepare_tag_handle(self, handle):
+        if not handle:
+            raise EmitterError("tag handle must not be empty")
+        if handle[0] != u'!' or handle[-1] != u'!':
+            raise EmitterError("tag handle must start and end with '!': %r"
+                    % (handle.encode('utf-8')))
+        for ch in handle[1:-1]:
+            if not (u'0' <= ch <= u'9' or u'A' <= ch <= u'Z' or u'a' <= ch <= u'z'  \
+                    or ch in u'-_'):
+                raise EmitterError("invalid character %r in the tag handle: %r"
+                        % (ch.encode('utf-8'), handle.encode('utf-8')))
+        return handle
+
+    def prepare_tag_prefix(self, prefix):
+        if not prefix:
+            raise EmitterError("tag prefix must not be empty")
+        chunks = []
+        start = end = 0
+        if prefix[0] == u'!':
+            end = 1
+        while end < len(prefix):
+            ch = prefix[end]
+            if u'0' <= ch <= u'9' or u'A' <= ch <= u'Z' or u'a' <= ch <= u'z'   \
+                    or ch in u'-;/?!:@&=+$,_.~*\'()[]':
+                end += 1
+            else:
+                if start < end:
+                    chunks.append(prefix[start:end])
+                start = end = end+1
+                data = ch.encode('utf-8')
+                for ch in data:
+                    chunks.append(u'%%%02X' % ord(ch))
+        if start < end:
+            chunks.append(prefix[start:end])
+        return u''.join(chunks)
+
+    def prepare_tag(self, tag):
+        if not tag:
+            raise EmitterError("tag must not be empty")
+        if tag == u'!':
+            return tag
+        handle = None
+        suffix = tag
+        prefixes = self.tag_prefixes.keys()
+        prefixes.sort()
+        for prefix in prefixes:
+            if tag.startswith(prefix)   \
+                    and (prefix == u'!' or len(prefix) < len(tag)):
+                handle = self.tag_prefixes[prefix]
+                suffix = tag[len(prefix):]
+        chunks = []
+        start = end = 0
+        while end < len(suffix):
+            ch = suffix[end]
+            if u'0' <= ch <= u'9' or u'A' <= ch <= u'Z' or u'a' <= ch <= u'z'   \
+                    or ch in u'-;/?:@&=+$,_.~*\'()[]'   \
+                    or (ch == u'!' and handle != u'!'):
+                end += 1
+            else:
+                if start < end:
+                    chunks.append(suffix[start:end])
+                start = end = end+1
+                data = ch.encode('utf-8')
+                for ch in data:
+                    chunks.append(u'%%%02X' % ord(ch))
+        if start < end:
+            chunks.append(suffix[start:end])
+        suffix_text = u''.join(chunks)
+        if handle:
+            return u'%s%s' % (handle, suffix_text)
+        else:
+            return u'!<%s>' % suffix_text
+
+    def prepare_anchor(self, anchor):
+        if not anchor:
+            raise EmitterError("anchor must not be empty")
+        for ch in anchor:
+            if not (u'0' <= ch <= u'9' or u'A' <= ch <= u'Z' or u'a' <= ch <= u'z'  \
+                    or ch in u'-_'):
+                raise EmitterError("invalid character %r in the anchor: %r"
+                        % (ch.encode('utf-8'), anchor.encode('utf-8')))
+        return anchor
+
+    def analyze_scalar(self, scalar):
+
+        # Empty scalar is a special case.
+        if not scalar:
+            return ScalarAnalysis(scalar=scalar, empty=True, multiline=False,
+                    allow_flow_plain=False, allow_block_plain=True,
+                    allow_single_quoted=True, allow_double_quoted=True,
+                    allow_block=False)
+
+        # Indicators and special characters.
+        block_indicators = False
+        flow_indicators = False
+        line_breaks = False
+        special_characters = False
+
+        # Important whitespace combinations.
+        leading_space = False
+        leading_break = False
+        trailing_space = False
+        trailing_break = False
+        break_space = False
+        space_break = False
+
+        # Check document indicators.
+        if scalar.startswith(u'---') or scalar.startswith(u'...'):
+            block_indicators = True
+            flow_indicators = True
+
+        # First character or preceded by a whitespace.
+        preceeded_by_whitespace = True
+
+        # Last character or followed by a whitespace.
+        followed_by_whitespace = (len(scalar) == 1 or
+                scalar[1] in u'\0 \t\r\n\x85\u2028\u2029')
+
+        # The previous character is a space.
+        previous_space = False
+
+        # The previous character is a break.
+        previous_break = False
+
+        index = 0
+        while index < len(scalar):
+            ch = scalar[index]
+
+            # Check for indicators.
+            if index == 0:
+                # Leading indicators are special characters.
+                if ch in u'#,[]{}&*!|>\'\"%@`': 
+                    flow_indicators = True
+                    block_indicators = True
+                if ch in u'?:':
+                    flow_indicators = True
+                    if followed_by_whitespace:
+                        block_indicators = True
+                if ch == u'-' and followed_by_whitespace:
+                    flow_indicators = True
+                    block_indicators = True
+            else:
+                # Some indicators cannot appear within a scalar as well.
+                if ch in u',?[]{}':
+                    flow_indicators = True
+                if ch == u':':
+                    flow_indicators = True
+                    if followed_by_whitespace:
+                        block_indicators = True
+                if ch == u'#' and preceeded_by_whitespace:
+                    flow_indicators = True
+                    block_indicators = True
+
+            # Check for line breaks, special, and unicode characters.
+            if ch in u'\n\x85\u2028\u2029':
+                line_breaks = True
+            if not (ch == u'\n' or u'\x20' <= ch <= u'\x7E'):
+                if (ch == u'\x85' or u'\xA0' <= ch <= u'\uD7FF'
+                        or u'\uE000' <= ch <= u'\uFFFD') and ch != u'\uFEFF':
+                    unicode_characters = True
+                    if not self.allow_unicode:
+                        special_characters = True
+                else:
+                    special_characters = True
+
+            # Detect important whitespace combinations.
+            if ch == u' ':
+                if index == 0:
+                    leading_space = True
+                if index == len(scalar)-1:
+                    trailing_space = True
+                if previous_break:
+                    break_space = True
+                previous_space = True
+                previous_break = False
+            elif ch in u'\n\x85\u2028\u2029':
+                if index == 0:
+                    leading_break = True
+                if index == len(scalar)-1:
+                    trailing_break = True
+                if previous_space:
+                    space_break = True
+                previous_space = False
+                previous_break = True
+            else:
+                previous_space = False
+                previous_break = False
+
+            # Prepare for the next character.
+            index += 1
+            preceeded_by_whitespace = (ch in u'\0 \t\r\n\x85\u2028\u2029')
+            followed_by_whitespace = (index+1 >= len(scalar) or
+                    scalar[index+1] in u'\0 \t\r\n\x85\u2028\u2029')
+
+        # Let's decide what styles are allowed.
+        allow_flow_plain = True
+        allow_block_plain = True
+        allow_single_quoted = True
+        allow_double_quoted = True
+        allow_block = True
+
+        # Leading and trailing whitespaces are bad for plain scalars.
+        if (leading_space or leading_break
+                or trailing_space or trailing_break):
+            allow_flow_plain = allow_block_plain = False
+
+        # We do not permit trailing spaces for block scalars.
+        if trailing_space:
+            allow_block = False
+
+        # Spaces at the beginning of a new line are only acceptable for block
+        # scalars.
+        if break_space:
+            allow_flow_plain = allow_block_plain = allow_single_quoted = False
+
+        # Spaces followed by breaks, as well as special character are only
+        # allowed for double quoted scalars.
+        if space_break or special_characters:
+            allow_flow_plain = allow_block_plain =  \
+            allow_single_quoted = allow_block = False
+
+        # Although the plain scalar writer supports breaks, we never emit
+        # multiline plain scalars.
+        if line_breaks:
+            allow_flow_plain = allow_block_plain = False
+
+        # Flow indicators are forbidden for flow plain scalars.
+        if flow_indicators:
+            allow_flow_plain = False
+
+        # Block indicators are forbidden for block plain scalars.
+        if block_indicators:
+            allow_block_plain = False
+
+        return ScalarAnalysis(scalar=scalar,
+                empty=False, multiline=line_breaks,
+                allow_flow_plain=allow_flow_plain,
+                allow_block_plain=allow_block_plain,
+                allow_single_quoted=allow_single_quoted,
+                allow_double_quoted=allow_double_quoted,
+                allow_block=allow_block)
+
+    # Writers.
+
+    def flush_stream(self):
+        if hasattr(self.stream, 'flush'):
+            self.stream.flush()
+
+    def write_stream_start(self):
+        # Write BOM if needed.
+        if self.encoding and self.encoding.startswith('utf-16'):
+            self.stream.write(u'\uFEFF'.encode(self.encoding))
+
+    def write_stream_end(self):
+        self.flush_stream()
+
+    def write_indicator(self, indicator, need_whitespace,
+            whitespace=False, indention=False):
+        if self.whitespace or not need_whitespace:
+            data = indicator
+        else:
+            data = u' '+indicator
+        self.whitespace = whitespace
+        self.indention = self.indention and indention
+        self.column += len(data)
+        self.open_ended = False
+        if self.encoding:
+            data = data.encode(self.encoding)
+        self.stream.write(data)
+
+    def write_indent(self):
+        indent = self.indent or 0
+        if not self.indention or self.column > indent   \
+                or (self.column == indent and not self.whitespace):
+            self.write_line_break()
+        if self.column < indent:
+            self.whitespace = True
+            data = u' '*(indent-self.column)
+            self.column = indent
+            if self.encoding:
+                data = data.encode(self.encoding)
+            self.stream.write(data)
+
+    def write_line_break(self, data=None):
+        if data is None:
+            data = self.best_line_break
+        self.whitespace = True
+        self.indention = True
+        self.line += 1
+        self.column = 0
+        if self.encoding:
+            data = data.encode(self.encoding)
+        self.stream.write(data)
+
+    def write_version_directive(self, version_text):
+        data = u'%%YAML %s' % version_text
+        if self.encoding:
+            data = data.encode(self.encoding)
+        self.stream.write(data)
+        self.write_line_break()
+
+    def write_tag_directive(self, handle_text, prefix_text):
+        data = u'%%TAG %s %s' % (handle_text, prefix_text)
+        if self.encoding:
+            data = data.encode(self.encoding)
+        self.stream.write(data)
+        self.write_line_break()
+
+    # Scalar streams.
+
+    def write_single_quoted(self, text, split=True):
+        self.write_indicator(u'\'', True)
+        spaces = False
+        breaks = False
+        start = end = 0
+        while end <= len(text):
+            ch = None
+            if end < len(text):
+                ch = text[end]
+            if spaces:
+                if ch is None or ch != u' ':
+                    if start+1 == end and self.column > self.best_width and split   \
+                            and start != 0 and end != len(text):
+                        self.write_indent()
+                    else:
+                        data = text[start:end]
+                        self.column += len(data)
+                        if self.encoding:
+                            data = data.encode(self.encoding)
+                        self.stream.write(data)
+                    start = end
+            elif breaks:
+                if ch is None or ch not in u'\n\x85\u2028\u2029':
+                    if text[start] == u'\n':
+                        self.write_line_break()
+                    for br in text[start:end]:
+                        if br == u'\n':
+                            self.write_line_break()
+                        else:
+                            self.write_line_break(br)
+                    self.write_indent()
+                    start = end
+            else:
+                if ch is None or ch in u' \n\x85\u2028\u2029' or ch == u'\'':
+                    if start < end:
+                        data = text[start:end]
+                        self.column += len(data)
+                        if self.encoding:
+                            data = data.encode(self.encoding)
+                        self.stream.write(data)
+                        start = end
+            if ch == u'\'':
+                data = u'\'\''
+                self.column += 2
+                if self.encoding:
+                    data = data.encode(self.encoding)
+                self.stream.write(data)
+                start = end + 1
+            if ch is not None:
+                spaces = (ch == u' ')
+                breaks = (ch in u'\n\x85\u2028\u2029')
+            end += 1
+        self.write_indicator(u'\'', False)
+
+    ESCAPE_REPLACEMENTS = {
+        u'\0':      u'0',
+        u'\x07':    u'a',
+        u'\x08':    u'b',
+        u'\x09':    u't',
+        u'\x0A':    u'n',
+        u'\x0B':    u'v',
+        u'\x0C':    u'f',
+        u'\x0D':    u'r',
+        u'\x1B':    u'e',
+        u'\"':      u'\"',
+        u'\\':      u'\\',
+        u'\x85':    u'N',
+        u'\xA0':    u'_',
+        u'\u2028':  u'L',
+        u'\u2029':  u'P',
+    }
+
+    def write_double_quoted(self, text, split=True):
+        self.write_indicator(u'"', True)
+        start = end = 0
+        while end <= len(text):
+            ch = None
+            if end < len(text):
+                ch = text[end]
+            if ch is None or ch in u'"\\\x85\u2028\u2029\uFEFF' \
+                    or not (u'\x20' <= ch <= u'\x7E'
+                        or (self.allow_unicode
+                            and (u'\xA0' <= ch <= u'\uD7FF'
+                                or u'\uE000' <= ch <= u'\uFFFD'))):
+                if start < end:
+                    data = text[start:end]
+                    self.column += len(data)
+                    if self.encoding:
+                        data = data.encode(self.encoding)
+                    self.stream.write(data)
+                    start = end
+                if ch is not None:
+                    if ch in self.ESCAPE_REPLACEMENTS:
+                        data = u'\\'+self.ESCAPE_REPLACEMENTS[ch]
+                    elif ch <= u'\xFF':
+                        data = u'\\x%02X' % ord(ch)
+                    elif ch <= u'\uFFFF':
+                        data = u'\\u%04X' % ord(ch)
+                    else:
+                        data = u'\\U%08X' % ord(ch)
+                    self.column += len(data)
+                    if self.encoding:
+                        data = data.encode(self.encoding)
+                    self.stream.write(data)
+                    start = end+1
+            if 0 < end < len(text)-1 and (ch == u' ' or start >= end)   \
+                    and self.column+(end-start) > self.best_width and split:
+                data = text[start:end]+u'\\'
+                if start < end:
+                    start = end
+                self.column += len(data)
+                if self.encoding:
+                    data = data.encode(self.encoding)
+                self.stream.write(data)
+                self.write_indent()
+                self.whitespace = False
+                self.indention = False
+                if text[start] == u' ':
+                    data = u'\\'
+                    self.column += len(data)
+                    if self.encoding:
+                        data = data.encode(self.encoding)
+                    self.stream.write(data)
+            end += 1
+        self.write_indicator(u'"', False)
+
+    def determine_block_hints(self, text):
+        hints = u''
+        if text:
+            if text[0] in u' \n\x85\u2028\u2029':
+                hints += unicode(self.best_indent)
+            if text[-1] not in u'\n\x85\u2028\u2029':
+                hints += u'-'
+            elif len(text) == 1 or text[-2] in u'\n\x85\u2028\u2029':
+                hints += u'+'
+        return hints
+
+    def write_folded(self, text):
+        hints = self.determine_block_hints(text)
+        self.write_indicator(u'>'+hints, True)
+        if hints[-1:] == u'+':
+            self.open_ended = True
+        self.write_line_break()
+        leading_space = True
+        spaces = False
+        breaks = True
+        start = end = 0
+        while end <= len(text):
+            ch = None
+            if end < len(text):
+                ch = text[end]
+            if breaks:
+                if ch is None or ch not in u'\n\x85\u2028\u2029':
+                    if not leading_space and ch is not None and ch != u' '  \
+                            and text[start] == u'\n':
+                        self.write_line_break()
+                    leading_space = (ch == u' ')
+                    for br in text[start:end]:
+                        if br == u'\n':
+                            self.write_line_break()
+                        else:
+                            self.write_line_break(br)
+                    if ch is not None:
+                        self.write_indent()
+                    start = end
+            elif spaces:
+                if ch != u' ':
+                    if start+1 == end and self.column > self.best_width:
+                        self.write_indent()
+                    else:
+                        data = text[start:end]
+                        self.column += len(data)
+                        if self.encoding:
+                            data = data.encode(self.encoding)
+                        self.stream.write(data)
+                    start = end
+            else:
+                if ch is None or ch in u' \n\x85\u2028\u2029':
+                    data = text[start:end]
+                    self.column += len(data)
+                    if self.encoding:
+                        data = data.encode(self.encoding)
+                    self.stream.write(data)
+                    if ch is None:
+                        self.write_line_break()
+                    start = end
+            if ch is not None:
+                breaks = (ch in u'\n\x85\u2028\u2029')
+                spaces = (ch == u' ')
+            end += 1
+
+    def write_literal(self, text):
+        hints = self.determine_block_hints(text)
+        self.write_indicator(u'|'+hints, True)
+        if hints[-1:] == u'+':
+            self.open_ended = True
+        self.write_line_break()
+        breaks = True
+        start = end = 0
+        while end <= len(text):
+            ch = None
+            if end < len(text):
+                ch = text[end]
+            if breaks:
+                if ch is None or ch not in u'\n\x85\u2028\u2029':
+                    for br in text[start:end]:
+                        if br == u'\n':
+                            self.write_line_break()
+                        else:
+                            self.write_line_break(br)
+                    if ch is not None:
+                        self.write_indent()
+                    start = end
+            else:
+                if ch is None or ch in u'\n\x85\u2028\u2029':
+                    data = text[start:end]
+                    if self.encoding:
+                        data = data.encode(self.encoding)
+                    self.stream.write(data)
+                    if ch is None:
+                        self.write_line_break()
+                    start = end
+            if ch is not None:
+                breaks = (ch in u'\n\x85\u2028\u2029')
+            end += 1
+
+    def write_plain(self, text, split=True):
+        if self.root_context:
+            self.open_ended = True
+        if not text:
+            return
+        if not self.whitespace:
+            data = u' '
+            self.column += len(data)
+            if self.encoding:
+                data = data.encode(self.encoding)
+            self.stream.write(data)
+        self.whitespace = False
+        self.indention = False
+        spaces = False
+        breaks = False
+        start = end = 0
+        while end <= len(text):
+            ch = None
+            if end < len(text):
+                ch = text[end]
+            if spaces:
+                if ch != u' ':
+                    if start+1 == end and self.column > self.best_width and split:
+                        self.write_indent()
+                        self.whitespace = False
+                        self.indention = False
+                    else:
+                        data = text[start:end]
+                        self.column += len(data)
+                        if self.encoding:
+                            data = data.encode(self.encoding)
+                        self.stream.write(data)
+                    start = end
+            elif breaks:
+                if ch not in u'\n\x85\u2028\u2029':
+                    if text[start] == u'\n':
+                        self.write_line_break()
+                    for br in text[start:end]:
+                        if br == u'\n':
+                            self.write_line_break()
+                        else:
+                            self.write_line_break(br)
+                    self.write_indent()
+                    self.whitespace = False
+                    self.indention = False
+                    start = end
+            else:
+                if ch is None or ch in u' \n\x85\u2028\u2029':
+                    data = text[start:end]
+                    self.column += len(data)
+                    if self.encoding:
+                        data = data.encode(self.encoding)
+                    self.stream.write(data)
+                    start = end
+            if ch is not None:
+                spaces = (ch == u' ')
+                breaks = (ch in u'\n\x85\u2028\u2029')
+            end += 1
+
diff --git a/lib/spack/external/yaml/error.py b/lib/spack/external/yaml/error.py
new file mode 100644
index 0000000000..577686db5f
--- /dev/null
+++ b/lib/spack/external/yaml/error.py
@@ -0,0 +1,75 @@
+
+__all__ = ['Mark', 'YAMLError', 'MarkedYAMLError']
+
+class Mark(object):
+
+    def __init__(self, name, index, line, column, buffer, pointer):
+        self.name = name
+        self.index = index
+        self.line = line
+        self.column = column
+        self.buffer = buffer
+        self.pointer = pointer
+
+    def get_snippet(self, indent=4, max_length=75):
+        if self.buffer is None:
+            return None
+        head = ''
+        start = self.pointer
+        while start > 0 and self.buffer[start-1] not in u'\0\r\n\x85\u2028\u2029':
+            start -= 1
+            if self.pointer-start > max_length/2-1:
+                head = ' ... '
+                start += 5
+                break
+        tail = ''
+        end = self.pointer
+        while end < len(self.buffer) and self.buffer[end] not in u'\0\r\n\x85\u2028\u2029':
+            end += 1
+            if end-self.pointer > max_length/2-1:
+                tail = ' ... '
+                end -= 5
+                break
+        snippet = self.buffer[start:end].encode('utf-8')
+        return ' '*indent + head + snippet + tail + '\n'  \
+                + ' '*(indent+self.pointer-start+len(head)) + '^'
+
+    def __str__(self):
+        snippet = self.get_snippet()
+        where = "  in \"%s\", line %d, column %d"   \
+                % (self.name, self.line+1, self.column+1)
+        if snippet is not None:
+            where += ":\n"+snippet
+        return where
+
+class YAMLError(Exception):
+    pass
+
+class MarkedYAMLError(YAMLError):
+
+    def __init__(self, context=None, context_mark=None,
+            problem=None, problem_mark=None, note=None):
+        self.context = context
+        self.context_mark = context_mark
+        self.problem = problem
+        self.problem_mark = problem_mark
+        self.note = note
+
+    def __str__(self):
+        lines = []
+        if self.context is not None:
+            lines.append(self.context)
+        if self.context_mark is not None  \
+            and (self.problem is None or self.problem_mark is None
+                    or self.context_mark.name != self.problem_mark.name
+                    or self.context_mark.line != self.problem_mark.line
+                    or self.context_mark.column != self.problem_mark.column):
+            lines.append(str(self.context_mark))
+        if self.problem is not None:
+            lines.append(self.problem)
+        if self.problem_mark is not None:
+            lines.append(str(self.problem_mark))
+        if self.note is not None:
+            lines.append(self.note)
+        return '\n'.join(lines)
+
diff --git a/lib/spack/external/yaml/events.py b/lib/spack/external/yaml/events.py
new file mode 100644
index 0000000000..f79ad389cb
--- /dev/null
+++ b/lib/spack/external/yaml/events.py
@@ -0,0 +1,86 @@
+
+# Abstract classes.
+
+class Event(object):
+    def __init__(self, start_mark=None, end_mark=None):
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+    def __repr__(self):
+        attributes = [key for key in ['anchor', 'tag', 'implicit', 'value']
+                if hasattr(self, key)]
+        arguments = ', '.join(['%s=%r' % (key, getattr(self, key))
+                for key in attributes])
+        return '%s(%s)' % (self.__class__.__name__, arguments)
+
+class NodeEvent(Event):
+    def __init__(self, anchor, start_mark=None, end_mark=None):
+        self.anchor = anchor
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+
+class CollectionStartEvent(NodeEvent):
+    def __init__(self, anchor, tag, implicit, start_mark=None, end_mark=None,
+            flow_style=None):
+        self.anchor = anchor
+        self.tag = tag
+        self.implicit = implicit
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+        self.flow_style = flow_style
+
+class CollectionEndEvent(Event):
+    pass
+
+# Implementations.
+
+class StreamStartEvent(Event):
+    def __init__(self, start_mark=None, end_mark=None, encoding=None):
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+        self.encoding = encoding
+
+class StreamEndEvent(Event):
+    pass
+
+class DocumentStartEvent(Event):
+    def __init__(self, start_mark=None, end_mark=None,
+            explicit=None, version=None, tags=None):
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+        self.explicit = explicit
+        self.version = version
+        self.tags = tags
+
+class DocumentEndEvent(Event):
+    def __init__(self, start_mark=None, end_mark=None,
+            explicit=None):
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+        self.explicit = explicit
+
+class AliasEvent(NodeEvent):
+    pass
+
+class ScalarEvent(NodeEvent):
+    def __init__(self, anchor, tag, implicit, value,
+            start_mark=None, end_mark=None, style=None):
+        self.anchor = anchor
+        self.tag = tag
+        self.implicit = implicit
+        self.value = value
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+        self.style = style
+
+class SequenceStartEvent(CollectionStartEvent):
+    pass
+
+class SequenceEndEvent(CollectionEndEvent):
+    pass
+
+class MappingStartEvent(CollectionStartEvent):
+    pass
+
+class MappingEndEvent(CollectionEndEvent):
+    pass
+
diff --git a/lib/spack/external/yaml/loader.py b/lib/spack/external/yaml/loader.py
new file mode 100644
index 0000000000..293ff467b1
--- /dev/null
+++ b/lib/spack/external/yaml/loader.py
@@ -0,0 +1,40 @@
+
+__all__ = ['BaseLoader', 'SafeLoader', 'Loader']
+
+from reader import *
+from scanner import *
+from parser import *
+from composer import *
+from constructor import *
+from resolver import *
+
+class BaseLoader(Reader, Scanner, Parser, Composer, BaseConstructor, BaseResolver):
+
+    def __init__(self, stream):
+        Reader.__init__(self, stream)
+        Scanner.__init__(self)
+        Parser.__init__(self)
+        Composer.__init__(self)
+        BaseConstructor.__init__(self)
+        BaseResolver.__init__(self)
+
+class SafeLoader(Reader, Scanner, Parser, Composer, SafeConstructor, Resolver):
+
+    def __init__(self, stream):
+        Reader.__init__(self, stream)
+        Scanner.__init__(self)
+        Parser.__init__(self)
+        Composer.__init__(self)
+        SafeConstructor.__init__(self)
+        Resolver.__init__(self)
+
+class Loader(Reader, Scanner, Parser, Composer, Constructor, Resolver):
+
+    def __init__(self, stream):
+        Reader.__init__(self, stream)
+        Scanner.__init__(self)
+        Parser.__init__(self)
+        Composer.__init__(self)
+        Constructor.__init__(self)
+        Resolver.__init__(self)
+
diff --git a/lib/spack/external/yaml/nodes.py b/lib/spack/external/yaml/nodes.py
new file mode 100644
index 0000000000..c4f070c41e
--- /dev/null
+++ b/lib/spack/external/yaml/nodes.py
@@ -0,0 +1,49 @@
+
+class Node(object):
+    def __init__(self, tag, value, start_mark, end_mark):
+        self.tag = tag
+        self.value = value
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+    def __repr__(self):
+        value = self.value
+        #if isinstance(value, list):
+        #    if len(value) == 0:
+        #        value = '<empty>'
+        #    elif len(value) == 1:
+        #        value = '<1 item>'
+        #    else:
+        #        value = '<%d items>' % len(value)
+        #else:
+        #    if len(value) > 75:
+        #        value = repr(value[:70]+u' ... ')
+        #    else:
+        #        value = repr(value)
+        value = repr(value)
+        return '%s(tag=%r, value=%s)' % (self.__class__.__name__, self.tag, value)
+
+class ScalarNode(Node):
+    id = 'scalar'
+    def __init__(self, tag, value,
+            start_mark=None, end_mark=None, style=None):
+        self.tag = tag
+        self.value = value
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+        self.style = style
+
+class CollectionNode(Node):
+    def __init__(self, tag, value,
+            start_mark=None, end_mark=None, flow_style=None):
+        self.tag = tag
+        self.value = value
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+        self.flow_style = flow_style
+
+class SequenceNode(CollectionNode):
+    id = 'sequence'
+
+class MappingNode(CollectionNode):
+    id = 'mapping'
+
diff --git a/lib/spack/external/yaml/parser.py b/lib/spack/external/yaml/parser.py
new file mode 100644
index 0000000000..f9e3057f33
--- /dev/null
+++ b/lib/spack/external/yaml/parser.py
@@ -0,0 +1,589 @@
+
+# The following YAML grammar is LL(1) and is parsed by a recursive descent
+# parser.
+#
+# stream            ::= STREAM-START implicit_document? explicit_document* STREAM-END
+# implicit_document ::= block_node DOCUMENT-END*
+# explicit_document ::= DIRECTIVE* DOCUMENT-START block_node? DOCUMENT-END*
+# block_node_or_indentless_sequence ::=
+#                       ALIAS
+#                       | properties (block_content | indentless_block_sequence)?
+#                       | block_content
+#                       | indentless_block_sequence
+# block_node        ::= ALIAS
+#                       | properties block_content?
+#                       | block_content
+# flow_node         ::= ALIAS
+#                       | properties flow_content?
+#                       | flow_content
+# properties        ::= TAG ANCHOR? | ANCHOR TAG?
+# block_content     ::= block_collection | flow_collection | SCALAR
+# flow_content      ::= flow_collection | SCALAR
+# block_collection  ::= block_sequence | block_mapping
+# flow_collection   ::= flow_sequence | flow_mapping
+# block_sequence    ::= BLOCK-SEQUENCE-START (BLOCK-ENTRY block_node?)* BLOCK-END
+# indentless_sequence   ::= (BLOCK-ENTRY block_node?)+
+# block_mapping     ::= BLOCK-MAPPING_START
+#                       ((KEY block_node_or_indentless_sequence?)?
+#                       (VALUE block_node_or_indentless_sequence?)?)*
+#                       BLOCK-END
+# flow_sequence     ::= FLOW-SEQUENCE-START
+#                       (flow_sequence_entry FLOW-ENTRY)*
+#                       flow_sequence_entry?
+#                       FLOW-SEQUENCE-END
+# flow_sequence_entry   ::= flow_node | KEY flow_node? (VALUE flow_node?)?
+# flow_mapping      ::= FLOW-MAPPING-START
+#                       (flow_mapping_entry FLOW-ENTRY)*
+#                       flow_mapping_entry?
+#                       FLOW-MAPPING-END
+# flow_mapping_entry    ::= flow_node | KEY flow_node? (VALUE flow_node?)?
+#
+# FIRST sets:
+#
+# stream: { STREAM-START }
+# explicit_document: { DIRECTIVE DOCUMENT-START }
+# implicit_document: FIRST(block_node)
+# block_node: { ALIAS TAG ANCHOR SCALAR BLOCK-SEQUENCE-START BLOCK-MAPPING-START FLOW-SEQUENCE-START FLOW-MAPPING-START }
+# flow_node: { ALIAS ANCHOR TAG SCALAR FLOW-SEQUENCE-START FLOW-MAPPING-START }
+# block_content: { BLOCK-SEQUENCE-START BLOCK-MAPPING-START FLOW-SEQUENCE-START FLOW-MAPPING-START SCALAR }
+# flow_content: { FLOW-SEQUENCE-START FLOW-MAPPING-START SCALAR }
+# block_collection: { BLOCK-SEQUENCE-START BLOCK-MAPPING-START }
+# flow_collection: { FLOW-SEQUENCE-START FLOW-MAPPING-START }
+# block_sequence: { BLOCK-SEQUENCE-START }
+# block_mapping: { BLOCK-MAPPING-START }
+# block_node_or_indentless_sequence: { ALIAS ANCHOR TAG SCALAR BLOCK-SEQUENCE-START BLOCK-MAPPING-START FLOW-SEQUENCE-START FLOW-MAPPING-START BLOCK-ENTRY }
+# indentless_sequence: { ENTRY }
+# flow_collection: { FLOW-SEQUENCE-START FLOW-MAPPING-START }
+# flow_sequence: { FLOW-SEQUENCE-START }
+# flow_mapping: { FLOW-MAPPING-START }
+# flow_sequence_entry: { ALIAS ANCHOR TAG SCALAR FLOW-SEQUENCE-START FLOW-MAPPING-START KEY }
+# flow_mapping_entry: { ALIAS ANCHOR TAG SCALAR FLOW-SEQUENCE-START FLOW-MAPPING-START KEY }
+
+__all__ = ['Parser', 'ParserError']
+
+from error import MarkedYAMLError
+from tokens import *
+from events import *
+from scanner import *
+
+class ParserError(MarkedYAMLError):
+    pass
+
+class Parser(object):
+    # Since writing a recursive-descendant parser is a straightforward task, we
+    # do not give many comments here.
+
+    DEFAULT_TAGS = {
+        u'!':   u'!',
+        u'!!':  u'tag:yaml.org,2002:',
+    }
+
+    def __init__(self):
+        self.current_event = None
+        self.yaml_version = None
+        self.tag_handles = {}
+        self.states = []
+        self.marks = []
+        self.state = self.parse_stream_start
+
+    def dispose(self):
+        # Reset the state attributes (to clear self-references)
+        self.states = []
+        self.state = None
+
+    def check_event(self, *choices):
+        # Check the type of the next event.
+        if self.current_event is None:
+            if self.state:
+                self.current_event = self.state()
+        if self.current_event is not None:
+            if not choices:
+                return True
+            for choice in choices:
+                if isinstance(self.current_event, choice):
+                    return True
+        return False
+
+    def peek_event(self):
+        # Get the next event.
+        if self.current_event is None:
+            if self.state:
+                self.current_event = self.state()
+        return self.current_event
+
+    def get_event(self):
+        # Get the next event and proceed further.
+        if self.current_event is None:
+            if self.state:
+                self.current_event = self.state()
+        value = self.current_event
+        self.current_event = None
+        return value
+
+    # stream    ::= STREAM-START implicit_document? explicit_document* STREAM-END
+    # implicit_document ::= block_node DOCUMENT-END*
+    # explicit_document ::= DIRECTIVE* DOCUMENT-START block_node? DOCUMENT-END*
+
+    def parse_stream_start(self):
+
+        # Parse the stream start.
+        token = self.get_token()
+        event = StreamStartEvent(token.start_mark, token.end_mark,
+                encoding=token.encoding)
+
+        # Prepare the next state.
+        self.state = self.parse_implicit_document_start
+
+        return event
+
+    def parse_implicit_document_start(self):
+
+        # Parse an implicit document.
+        if not self.check_token(DirectiveToken, DocumentStartToken,
+                StreamEndToken):
+            self.tag_handles = self.DEFAULT_TAGS
+            token = self.peek_token()
+            start_mark = end_mark = token.start_mark
+            event = DocumentStartEvent(start_mark, end_mark,
+                    explicit=False)
+
+            # Prepare the next state.
+            self.states.append(self.parse_document_end)
+            self.state = self.parse_block_node
+
+            return event
+
+        else:
+            return self.parse_document_start()
+
+    def parse_document_start(self):
+
+        # Parse any extra document end indicators.
+        while self.check_token(DocumentEndToken):
+            self.get_token()
+
+        # Parse an explicit document.
+        if not self.check_token(StreamEndToken):
+            token = self.peek_token()
+            start_mark = token.start_mark
+            version, tags = self.process_directives()
+            if not self.check_token(DocumentStartToken):
+                raise ParserError(None, None,
+                        "expected '<document start>', but found %r"
+                        % self.peek_token().id,
+                        self.peek_token().start_mark)
+            token = self.get_token()
+            end_mark = token.end_mark
+            event = DocumentStartEvent(start_mark, end_mark,
+                    explicit=True, version=version, tags=tags)
+            self.states.append(self.parse_document_end)
+            self.state = self.parse_document_content
+        else:
+            # Parse the end of the stream.
+            token = self.get_token()
+            event = StreamEndEvent(token.start_mark, token.end_mark)
+            assert not self.states
+            assert not self.marks
+            self.state = None
+        return event
+
+    def parse_document_end(self):
+
+        # Parse the document end.
+        token = self.peek_token()
+        start_mark = end_mark = token.start_mark
+        explicit = False
+        if self.check_token(DocumentEndToken):
+            token = self.get_token()
+            end_mark = token.end_mark
+            explicit = True
+        event = DocumentEndEvent(start_mark, end_mark,
+                explicit=explicit)
+
+        # Prepare the next state.
+        self.state = self.parse_document_start
+
+        return event
+
+    def parse_document_content(self):
+        if self.check_token(DirectiveToken,
+                DocumentStartToken, DocumentEndToken, StreamEndToken):
+            event = self.process_empty_scalar(self.peek_token().start_mark)
+            self.state = self.states.pop()
+            return event
+        else:
+            return self.parse_block_node()
+
+    def process_directives(self):
+        self.yaml_version = None
+        self.tag_handles = {}
+        while self.check_token(DirectiveToken):
+            token = self.get_token()
+            if token.name == u'YAML':
+                if self.yaml_version is not None:
+                    raise ParserError(None, None,
+                            "found duplicate YAML directive", token.start_mark)
+                major, minor = token.value
+                if major != 1:
+                    raise ParserError(None, None,
+                            "found incompatible YAML document (version 1.* is required)",
+                            token.start_mark)
+                self.yaml_version = token.value
+            elif token.name == u'TAG':
+                handle, prefix = token.value
+                if handle in self.tag_handles:
+                    raise ParserError(None, None,
+                            "duplicate tag handle %r" % handle.encode('utf-8'),
+                            token.start_mark)
+                self.tag_handles[handle] = prefix
+        if self.tag_handles:
+            value = self.yaml_version, self.tag_handles.copy()
+        else:
+            value = self.yaml_version, None
+        for key in self.DEFAULT_TAGS:
+            if key not in self.tag_handles:
+                self.tag_handles[key] = self.DEFAULT_TAGS[key]
+        return value
+
+    # block_node_or_indentless_sequence ::= ALIAS
+    #               | properties (block_content | indentless_block_sequence)?
+    #               | block_content
+    #               | indentless_block_sequence
+    # block_node    ::= ALIAS
+    #                   | properties block_content?
+    #                   | block_content
+    # flow_node     ::= ALIAS
+    #                   | properties flow_content?
+    #                   | flow_content
+    # properties    ::= TAG ANCHOR? | ANCHOR TAG?
+    # block_content     ::= block_collection | flow_collection | SCALAR
+    # flow_content      ::= flow_collection | SCALAR
+    # block_collection  ::= block_sequence | block_mapping
+    # flow_collection   ::= flow_sequence | flow_mapping
+
+    def parse_block_node(self):
+        return self.parse_node(block=True)
+
+    def parse_flow_node(self):
+        return self.parse_node()
+
+    def parse_block_node_or_indentless_sequence(self):
+        return self.parse_node(block=True, indentless_sequence=True)
+
+    def parse_node(self, block=False, indentless_sequence=False):
+        if self.check_token(AliasToken):
+            token = self.get_token()
+            event = AliasEvent(token.value, token.start_mark, token.end_mark)
+            self.state = self.states.pop()
+        else:
+            anchor = None
+            tag = None
+            start_mark = end_mark = tag_mark = None
+            if self.check_token(AnchorToken):
+                token = self.get_token()
+                start_mark = token.start_mark
+                end_mark = token.end_mark
+                anchor = token.value
+                if self.check_token(TagToken):
+                    token = self.get_token()
+                    tag_mark = token.start_mark
+                    end_mark = token.end_mark
+                    tag = token.value
+            elif self.check_token(TagToken):
+                token = self.get_token()
+                start_mark = tag_mark = token.start_mark
+                end_mark = token.end_mark
+                tag = token.value
+                if self.check_token(AnchorToken):
+                    token = self.get_token()
+                    end_mark = token.end_mark
+                    anchor = token.value
+            if tag is not None:
+                handle, suffix = tag
+                if handle is not None:
+                    if handle not in self.tag_handles:
+                        raise ParserError("while parsing a node", start_mark,
+                                "found undefined tag handle %r" % handle.encode('utf-8'),
+                                tag_mark)
+                    tag = self.tag_handles[handle]+suffix
+                else:
+                    tag = suffix
+            #if tag == u'!':
+            #    raise ParserError("while parsing a node", start_mark,
+            #            "found non-specific tag '!'", tag_mark,
+            #            "Please check 'http://pyyaml.org/wiki/YAMLNonSpecificTag' and share your opinion.")
+            if start_mark is None:
+                start_mark = end_mark = self.peek_token().start_mark
+            event = None
+            implicit = (tag is None or tag == u'!')
+            if indentless_sequence and self.check_token(BlockEntryToken):
+                end_mark = self.peek_token().end_mark
+                event = SequenceStartEvent(anchor, tag, implicit,
+                        start_mark, end_mark)
+                self.state = self.parse_indentless_sequence_entry
+            else:
+                if self.check_token(ScalarToken):
+                    token = self.get_token()
+                    end_mark = token.end_mark
+                    if (token.plain and tag is None) or tag == u'!':
+                        implicit = (True, False)
+                    elif tag is None:
+                        implicit = (False, True)
+                    else:
+                        implicit = (False, False)
+                    event = ScalarEvent(anchor, tag, implicit, token.value,
+                            start_mark, end_mark, style=token.style)
+                    self.state = self.states.pop()
+                elif self.check_token(FlowSequenceStartToken):
+                    end_mark = self.peek_token().end_mark
+                    event = SequenceStartEvent(anchor, tag, implicit,
+                            start_mark, end_mark, flow_style=True)
+                    self.state = self.parse_flow_sequence_first_entry
+                elif self.check_token(FlowMappingStartToken):
+                    end_mark = self.peek_token().end_mark
+                    event = MappingStartEvent(anchor, tag, implicit,
+                            start_mark, end_mark, flow_style=True)
+                    self.state = self.parse_flow_mapping_first_key
+                elif block and self.check_token(BlockSequenceStartToken):
+                    end_mark = self.peek_token().start_mark
+                    event = SequenceStartEvent(anchor, tag, implicit,
+                            start_mark, end_mark, flow_style=False)
+                    self.state = self.parse_block_sequence_first_entry
+                elif block and self.check_token(BlockMappingStartToken):
+                    end_mark = self.peek_token().start_mark
+                    event = MappingStartEvent(anchor, tag, implicit,
+                            start_mark, end_mark, flow_style=False)
+                    self.state = self.parse_block_mapping_first_key
+                elif anchor is not None or tag is not None:
+                    # Empty scalars are allowed even if a tag or an anchor is
+                    # specified.
+                    event = ScalarEvent(anchor, tag, (implicit, False), u'',
+                            start_mark, end_mark)
+                    self.state = self.states.pop()
+                else:
+                    if block:
+                        node = 'block'
+                    else:
+                        node = 'flow'
+                    token = self.peek_token()
+                    raise ParserError("while parsing a %s node" % node, start_mark,
+                            "expected the node content, but found %r" % token.id,
+                            token.start_mark)
+        return event
+
+    # block_sequence ::= BLOCK-SEQUENCE-START (BLOCK-ENTRY block_node?)* BLOCK-END
+
+    def parse_block_sequence_first_entry(self):
+        token = self.get_token()
+        self.marks.append(token.start_mark)
+        return self.parse_block_sequence_entry()
+
+    def parse_block_sequence_entry(self):
+        if self.check_token(BlockEntryToken):
+            token = self.get_token()
+            if not self.check_token(BlockEntryToken, BlockEndToken):
+                self.states.append(self.parse_block_sequence_entry)
+                return self.parse_block_node()
+            else:
+                self.state = self.parse_block_sequence_entry
+                return self.process_empty_scalar(token.end_mark)
+        if not self.check_token(BlockEndToken):
+            token = self.peek_token()
+            raise ParserError("while parsing a block collection", self.marks[-1],
+                    "expected <block end>, but found %r" % token.id, token.start_mark)
+        token = self.get_token()
+        event = SequenceEndEvent(token.start_mark, token.end_mark)
+        self.state = self.states.pop()
+        self.marks.pop()
+        return event
+
+    # indentless_sequence ::= (BLOCK-ENTRY block_node?)+
+
+    def parse_indentless_sequence_entry(self):
+        if self.check_token(BlockEntryToken):
+            token = self.get_token()
+            if not self.check_token(BlockEntryToken,
+                    KeyToken, ValueToken, BlockEndToken):
+                self.states.append(self.parse_indentless_sequence_entry)
+                return self.parse_block_node()
+            else:
+                self.state = self.parse_indentless_sequence_entry
+                return self.process_empty_scalar(token.end_mark)
+        token = self.peek_token()
+        event = SequenceEndEvent(token.start_mark, token.start_mark)
+        self.state = self.states.pop()
+        return event
+
+    # block_mapping     ::= BLOCK-MAPPING_START
+    #                       ((KEY block_node_or_indentless_sequence?)?
+    #                       (VALUE block_node_or_indentless_sequence?)?)*
+    #                       BLOCK-END
+
+    def parse_block_mapping_first_key(self):
+        token = self.get_token()
+        self.marks.append(token.start_mark)
+        return self.parse_block_mapping_key()
+
+    def parse_block_mapping_key(self):
+        if self.check_token(KeyToken):
+            token = self.get_token()
+            if not self.check_token(KeyToken, ValueToken, BlockEndToken):
+                self.states.append(self.parse_block_mapping_value)
+                return self.parse_block_node_or_indentless_sequence()
+            else:
+                self.state = self.parse_block_mapping_value
+                return self.process_empty_scalar(token.end_mark)
+        if not self.check_token(BlockEndToken):
+            token = self.peek_token()
+            raise ParserError("while parsing a block mapping", self.marks[-1],
+                    "expected <block end>, but found %r" % token.id, token.start_mark)
+        token = self.get_token()
+        event = MappingEndEvent(token.start_mark, token.end_mark)
+        self.state = self.states.pop()
+        self.marks.pop()
+        return event
+
+    def parse_block_mapping_value(self):
+        if self.check_token(ValueToken):
+            token = self.get_token()
+            if not self.check_token(KeyToken, ValueToken, BlockEndToken):
+                self.states.append(self.parse_block_mapping_key)
+                return self.parse_block_node_or_indentless_sequence()
+            else:
+                self.state = self.parse_block_mapping_key
+                return self.process_empty_scalar(token.end_mark)
+        else:
+            self.state = self.parse_block_mapping_key
+            token = self.peek_token()
+            return self.process_empty_scalar(token.start_mark)
+
+    # flow_sequence     ::= FLOW-SEQUENCE-START
+    #                       (flow_sequence_entry FLOW-ENTRY)*
+    #                       flow_sequence_entry?
+    #                       FLOW-SEQUENCE-END
+    # flow_sequence_entry   ::= flow_node | KEY flow_node? (VALUE flow_node?)?
+    #
+    # Note that while production rules for both flow_sequence_entry and
+    # flow_mapping_entry are equal, their interpretations are different.
+    # For `flow_sequence_entry`, the part `KEY flow_node? (VALUE flow_node?)?`
+    # generate an inline mapping (set syntax).
+
+    def parse_flow_sequence_first_entry(self):
+        token = self.get_token()
+        self.marks.append(token.start_mark)
+        return self.parse_flow_sequence_entry(first=True)
+
+    def parse_flow_sequence_entry(self, first=False):
+        if not self.check_token(FlowSequenceEndToken):
+            if not first:
+                if self.check_token(FlowEntryToken):
+                    self.get_token()
+                else:
+                    token = self.peek_token()
+                    raise ParserError("while parsing a flow sequence", self.marks[-1],
+                            "expected ',' or ']', but got %r" % token.id, token.start_mark)
+            
+            if self.check_token(KeyToken):
+                token = self.peek_token()
+                event = MappingStartEvent(None, None, True,
+                        token.start_mark, token.end_mark,
+                        flow_style=True)
+                self.state = self.parse_flow_sequence_entry_mapping_key
+                return event
+            elif not self.check_token(FlowSequenceEndToken):
+                self.states.append(self.parse_flow_sequence_entry)
+                return self.parse_flow_node()
+        token = self.get_token()
+        event = SequenceEndEvent(token.start_mark, token.end_mark)
+        self.state = self.states.pop()
+        self.marks.pop()
+        return event
+
+    def parse_flow_sequence_entry_mapping_key(self):
+        token = self.get_token()
+        if not self.check_token(ValueToken,
+                FlowEntryToken, FlowSequenceEndToken):
+            self.states.append(self.parse_flow_sequence_entry_mapping_value)
+            return self.parse_flow_node()
+        else:
+            self.state = self.parse_flow_sequence_entry_mapping_value
+            return self.process_empty_scalar(token.end_mark)
+
+    def parse_flow_sequence_entry_mapping_value(self):
+        if self.check_token(ValueToken):
+            token = self.get_token()
+            if not self.check_token(FlowEntryToken, FlowSequenceEndToken):
+                self.states.append(self.parse_flow_sequence_entry_mapping_end)
+                return self.parse_flow_node()
+            else:
+                self.state = self.parse_flow_sequence_entry_mapping_end
+                return self.process_empty_scalar(token.end_mark)
+        else:
+            self.state = self.parse_flow_sequence_entry_mapping_end
+            token = self.peek_token()
+            return self.process_empty_scalar(token.start_mark)
+
+    def parse_flow_sequence_entry_mapping_end(self):
+        self.state = self.parse_flow_sequence_entry
+        token = self.peek_token()
+        return MappingEndEvent(token.start_mark, token.start_mark)
+
+    # flow_mapping  ::= FLOW-MAPPING-START
+    #                   (flow_mapping_entry FLOW-ENTRY)*
+    #                   flow_mapping_entry?
+    #                   FLOW-MAPPING-END
+    # flow_mapping_entry    ::= flow_node | KEY flow_node? (VALUE flow_node?)?
+
+    def parse_flow_mapping_first_key(self):
+        token = self.get_token()
+        self.marks.append(token.start_mark)
+        return self.parse_flow_mapping_key(first=True)
+
+    def parse_flow_mapping_key(self, first=False):
+        if not self.check_token(FlowMappingEndToken):
+            if not first:
+                if self.check_token(FlowEntryToken):
+                    self.get_token()
+                else:
+                    token = self.peek_token()
+                    raise ParserError("while parsing a flow mapping", self.marks[-1],
+                            "expected ',' or '}', but got %r" % token.id, token.start_mark)
+            if self.check_token(KeyToken):
+                token = self.get_token()
+                if not self.check_token(ValueToken,
+                        FlowEntryToken, FlowMappingEndToken):
+                    self.states.append(self.parse_flow_mapping_value)
+                    return self.parse_flow_node()
+                else:
+                    self.state = self.parse_flow_mapping_value
+                    return self.process_empty_scalar(token.end_mark)
+            elif not self.check_token(FlowMappingEndToken):
+                self.states.append(self.parse_flow_mapping_empty_value)
+                return self.parse_flow_node()
+        token = self.get_token()
+        event = MappingEndEvent(token.start_mark, token.end_mark)
+        self.state = self.states.pop()
+        self.marks.pop()
+        return event
+
+    def parse_flow_mapping_value(self):
+        if self.check_token(ValueToken):
+            token = self.get_token()
+            if not self.check_token(FlowEntryToken, FlowMappingEndToken):
+                self.states.append(self.parse_flow_mapping_key)
+                return self.parse_flow_node()
+            else:
+                self.state = self.parse_flow_mapping_key
+                return self.process_empty_scalar(token.end_mark)
+        else:
+            self.state = self.parse_flow_mapping_key
+            token = self.peek_token()
+            return self.process_empty_scalar(token.start_mark)
+
+    def parse_flow_mapping_empty_value(self):
+        self.state = self.parse_flow_mapping_key
+        return self.process_empty_scalar(self.peek_token().start_mark)
+
+    def process_empty_scalar(self, mark):
+        return ScalarEvent(None, None, (True, False), u'', mark, mark)
+
diff --git a/lib/spack/external/yaml/reader.py b/lib/spack/external/yaml/reader.py
new file mode 100644
index 0000000000..a67af7c5da
--- /dev/null
+++ b/lib/spack/external/yaml/reader.py
@@ -0,0 +1,189 @@
+# This module contains abstractions for the input stream. You don't have to
+# looks further, there are no pretty code.
+#
+# We define two classes here.
+#
+#   Mark(source, line, column)
+# It's just a record and its only use is producing nice error messages.
+# Parser does not use it for any other purposes.
+#
+#   Reader(source, data)
+# Reader determines the encoding of `data` and converts it to unicode.
+# Reader provides the following methods and attributes:
+#   reader.peek(length=1) - return the next `length` characters
+#   reader.forward(length=1) - move the current position to `length` characters.
+#   reader.index - the number of the current character.
+#   reader.line, stream.column - the line and the column of the current character.
+
+__all__ = ['Reader', 'ReaderError']
+
+from error import YAMLError, Mark
+
+import codecs, re
+
+class ReaderError(YAMLError):
+
+    def __init__(self, name, position, character, encoding, reason):
+        self.name = name
+        self.character = character
+        self.position = position
+        self.encoding = encoding
+        self.reason = reason
+
+    def __str__(self):
+        if isinstance(self.character, str):
+            return "'%s' codec can't decode byte #x%02x: %s\n"  \
+                    "  in \"%s\", position %d"    \
+                    % (self.encoding, ord(self.character), self.reason,
+                            self.name, self.position)
+        else:
+            return "unacceptable character #x%04x: %s\n"    \
+                    "  in \"%s\", position %d"    \
+                    % (self.character, self.reason,
+                            self.name, self.position)
+
+class Reader(object):
+    # Reader:
+    # - determines the data encoding and converts it to unicode,
+    # - checks if characters are in allowed range,
+    # - adds '\0' to the end.
+
+    # Reader accepts
+    #  - a `str` object,
+    #  - a `unicode` object,
+    #  - a file-like object with its `read` method returning `str`,
+    #  - a file-like object with its `read` method returning `unicode`.
+
+    # Yeah, it's ugly and slow.
+
+    def __init__(self, stream, name=None):
+        self.stream = None
+        self.stream_pointer = 0
+        self.eof = True
+        self.buffer = u''
+        self.pointer = 0
+        self.raw_buffer = None
+        self.raw_decode = None
+        self.encoding = None
+        self.index = 0
+        self.line = 0
+        self.column = 0
+        if isinstance(stream, unicode):
+            self.name = "<unicode string>" if name is None else name
+            self.check_printable(stream)
+            self.buffer = stream+u'\0'
+        elif isinstance(stream, str):
+            self.name = "<string>" if name is None else name
+            self.raw_buffer = stream
+            self.determine_encoding()
+        else:
+            self.stream = stream
+            self.name = getattr(stream, 'name', "<file>") if name is None else name
+            self.eof = False
+            self.raw_buffer = ''
+            self.determine_encoding()
+
+    def peek(self, index=0):
+        try:
+            return self.buffer[self.pointer+index]
+        except IndexError:
+            self.update(index+1)
+            return self.buffer[self.pointer+index]
+
+    def prefix(self, length=1):
+        if self.pointer+length >= len(self.buffer):
+            self.update(length)
+        return self.buffer[self.pointer:self.pointer+length]
+
+    def forward(self, length=1):
+        if self.pointer+length+1 >= len(self.buffer):
+            self.update(length+1)
+        while length:
+            ch = self.buffer[self.pointer]
+            self.pointer += 1
+            self.index += 1
+            if ch in u'\n\x85\u2028\u2029'  \
+                    or (ch == u'\r' and self.buffer[self.pointer] != u'\n'):
+                self.line += 1
+                self.column = 0
+            elif ch != u'\uFEFF':
+                self.column += 1
+            length -= 1
+
+    def get_mark(self):
+        if self.stream is None:
+            return Mark(self.name, self.index, self.line, self.column,
+                    self.buffer, self.pointer)
+        else:
+            return Mark(self.name, self.index, self.line, self.column,
+                    None, None)
+
+    def determine_encoding(self):
+        while not self.eof and len(self.raw_buffer) < 2:
+            self.update_raw()
+        if not isinstance(self.raw_buffer, unicode):
+            if self.raw_buffer.startswith(codecs.BOM_UTF16_LE):
+                self.raw_decode = codecs.utf_16_le_decode
+                self.encoding = 'utf-16-le'
+            elif self.raw_buffer.startswith(codecs.BOM_UTF16_BE):
+                self.raw_decode = codecs.utf_16_be_decode
+                self.encoding = 'utf-16-be'
+            else:
+                self.raw_decode = codecs.utf_8_decode
+                self.encoding = 'utf-8'
+        self.update(1)
+
+    NON_PRINTABLE = re.compile(u'[^\x09\x0A\x0D\x20-\x7E\x85\xA0-\uD7FF\uE000-\uFFFD]')
+    def check_printable(self, data):
+        match = self.NON_PRINTABLE.search(data)
+        if match:
+            character = match.group()
+            position = self.index+(len(self.buffer)-self.pointer)+match.start()
+            raise ReaderError(self.name, position, ord(character),
+                    'unicode', "special characters are not allowed")
+
+    def update(self, length):
+        if self.raw_buffer is None:
+            return
+        self.buffer = self.buffer[self.pointer:]
+        self.pointer = 0
+        while len(self.buffer) < length:
+            if not self.eof:
+                self.update_raw()
+            if self.raw_decode is not None:
+                try:
+                    data, converted = self.raw_decode(self.raw_buffer,
+                            'strict', self.eof)
+                except UnicodeDecodeError, exc:
+                    character = exc.object[exc.start]
+                    if self.stream is not None:
+                        position = self.stream_pointer-len(self.raw_buffer)+exc.start
+                    else:
+                        position = exc.start
+                    raise ReaderError(self.name, position, character,
+                            exc.encoding, exc.reason)
+            else:
+                data = self.raw_buffer
+                converted = len(data)
+            self.check_printable(data)
+            self.buffer += data
+            self.raw_buffer = self.raw_buffer[converted:]
+            if self.eof:
+                self.buffer += u'\0'
+                self.raw_buffer = None
+                break
+
+    def update_raw(self, size=1024):
+        data = self.stream.read(size)
+        if data:
+            self.raw_buffer += data
+            self.stream_pointer += len(data)
+        else:
+            self.eof = True
+
+#try:
+#    import psyco
+#    psyco.bind(Reader)
+#except ImportError:
+#    pass
+
diff --git a/lib/spack/external/yaml/representer.py b/lib/spack/external/yaml/representer.py
new file mode 100644
index 0000000000..5f4fc70dbc
--- /dev/null
+++ b/lib/spack/external/yaml/representer.py
@@ -0,0 +1,484 @@
+
+__all__ = ['BaseRepresenter', 'SafeRepresenter', 'Representer',
+    'RepresenterError']
+
+from error import *
+from nodes import *
+
+import datetime
+
+import sys, copy_reg, types
+
+class RepresenterError(YAMLError):
+    pass
+
+class BaseRepresenter(object):
+
+    yaml_representers = {}
+    yaml_multi_representers = {}
+
+    def __init__(self, default_style=None, default_flow_style=None):
+        self.default_style = default_style
+        self.default_flow_style = default_flow_style
+        self.represented_objects = {}
+        self.object_keeper = []
+        self.alias_key = None
+
+    def represent(self, data):
+        node = self.represent_data(data)
+        self.serialize(node)
+        self.represented_objects = {}
+        self.object_keeper = []
+        self.alias_key = None
+
+    def get_classobj_bases(self, cls):
+        bases = [cls]
+        for base in cls.__bases__:
+            bases.extend(self.get_classobj_bases(base))
+        return bases
+
+    def represent_data(self, data):
+        if self.ignore_aliases(data):
+            self.alias_key = None
+        else:
+            self.alias_key = id(data)
+        if self.alias_key is not None:
+            if self.alias_key in self.represented_objects:
+                node = self.represented_objects[self.alias_key]
+                #if node is None:
+                #    raise RepresenterError("recursive objects are not allowed: %r" % data)
+                return node
+            #self.represented_objects[alias_key] = None
+            self.object_keeper.append(data)
+        data_types = type(data).__mro__
+        if type(data) is types.InstanceType:
+            data_types = self.get_classobj_bases(data.__class__)+list(data_types)
+        if data_types[0] in self.yaml_representers:
+            node = self.yaml_representers[data_types[0]](self, data)
+        else:
+            for data_type in data_types:
+                if data_type in self.yaml_multi_representers:
+                    node = self.yaml_multi_representers[data_type](self, data)
+                    break
+            else:
+                if None in self.yaml_multi_representers:
+                    node = self.yaml_multi_representers[None](self, data)
+                elif None in self.yaml_representers:
+                    node = self.yaml_representers[None](self, data)
+                else:
+                    node = ScalarNode(None, unicode(data))
+        #if alias_key is not None:
+        #    self.represented_objects[alias_key] = node
+        return node
+
+    def add_representer(cls, data_type, representer):
+        if not 'yaml_representers' in cls.__dict__:
+            cls.yaml_representers = cls.yaml_representers.copy()
+        cls.yaml_representers[data_type] = representer
+    add_representer = classmethod(add_representer)
+
+    def add_multi_representer(cls, data_type, representer):
+        if not 'yaml_multi_representers' in cls.__dict__:
+            cls.yaml_multi_representers = cls.yaml_multi_representers.copy()
+        cls.yaml_multi_representers[data_type] = representer
+    add_multi_representer = classmethod(add_multi_representer)
+
+    def represent_scalar(self, tag, value, style=None):
+        if style is None:
+            style = self.default_style
+        node = ScalarNode(tag, value, style=style)
+        if self.alias_key is not None:
+            self.represented_objects[self.alias_key] = node
+        return node
+
+    def represent_sequence(self, tag, sequence, flow_style=None):
+        value = []
+        node = SequenceNode(tag, value, flow_style=flow_style)
+        if self.alias_key is not None:
+            self.represented_objects[self.alias_key] = node
+        best_style = True
+        for item in sequence:
+            node_item = self.represent_data(item)
+            if not (isinstance(node_item, ScalarNode) and not node_item.style):
+                best_style = False
+            value.append(node_item)
+        if flow_style is None:
+            if self.default_flow_style is not None:
+                node.flow_style = self.default_flow_style
+            else:
+                node.flow_style = best_style
+        return node
+
+    def represent_mapping(self, tag, mapping, flow_style=None):
+        value = []
+        node = MappingNode(tag, value, flow_style=flow_style)
+        if self.alias_key is not None:
+            self.represented_objects[self.alias_key] = node
+        best_style = True
+        if hasattr(mapping, 'items'):
+            mapping = mapping.items()
+            mapping.sort()
+        for item_key, item_value in mapping:
+            node_key = self.represent_data(item_key)
+            node_value = self.represent_data(item_value)
+            if not (isinstance(node_key, ScalarNode) and not node_key.style):
+                best_style = False
+            if not (isinstance(node_value, ScalarNode) and not node_value.style):
+                best_style = False
+            value.append((node_key, node_value))
+        if flow_style is None:
+            if self.default_flow_style is not None:
+                node.flow_style = self.default_flow_style
+            else:
+                node.flow_style = best_style
+        return node
+
+    def ignore_aliases(self, data):
+        return False
+
+class SafeRepresenter(BaseRepresenter):
+
+    def ignore_aliases(self, data):
+        if data in [None, ()]:
+            return True
+        if isinstance(data, (str, unicode, bool, int, float)):
+            return True
+
+    def represent_none(self, data):
+        return self.represent_scalar(u'tag:yaml.org,2002:null',
+                u'null')
+
+    def represent_str(self, data):
+        tag = None
+        style = None
+        try:
+            data = unicode(data, 'ascii')
+            tag = u'tag:yaml.org,2002:str'
+        except UnicodeDecodeError:
+            try:
+                data = unicode(data, 'utf-8')
+                tag = u'tag:yaml.org,2002:str'
+            except UnicodeDecodeError:
+                data = data.encode('base64')
+                tag = u'tag:yaml.org,2002:binary'
+                style = '|'
+        return self.represent_scalar(tag, data, style=style)
+
+    def represent_unicode(self, data):
+        return self.represent_scalar(u'tag:yaml.org,2002:str', data)
+
+    def represent_bool(self, data):
+        if data:
+            value = u'true'
+        else:
+            value = u'false'
+        return self.represent_scalar(u'tag:yaml.org,2002:bool', value)
+
+    def represent_int(self, data):
+        return self.represent_scalar(u'tag:yaml.org,2002:int', unicode(data))
+
+    def represent_long(self, data):
+        return self.represent_scalar(u'tag:yaml.org,2002:int', unicode(data))
+
+    inf_value = 1e300
+    while repr(inf_value) != repr(inf_value*inf_value):
+        inf_value *= inf_value
+
+    def represent_float(self, data):
+        if data != data or (data == 0.0 and data == 1.0):
+            value = u'.nan'
+        elif data == self.inf_value:
+            value = u'.inf'
+        elif data == -self.inf_value:
+            value = u'-.inf'
+        else:
+            value = unicode(repr(data)).lower()
+            # Note that in some cases `repr(data)` represents a float number
+            # without the decimal parts.  For instance:
+            #   >>> repr(1e17)
+            #   '1e17'
+            # Unfortunately, this is not a valid float representation according
+            # to the definition of the `!!float` tag.  We fix this by adding
+            # '.0' before the 'e' symbol.
+            if u'.' not in value and u'e' in value:
+                value = value.replace(u'e', u'.0e', 1)
+        return self.represent_scalar(u'tag:yaml.org,2002:float', value)
+
+    def represent_list(self, data):
+        #pairs = (len(data) > 0 and isinstance(data, list))
+        #if pairs:
+        #    for item in data:
+        #        if not isinstance(item, tuple) or len(item) != 2:
+        #            pairs = False
+        #            break
+        #if not pairs:
+            return self.represent_sequence(u'tag:yaml.org,2002:seq', data)
+        #value = []
+        #for item_key, item_value in data:
+        #    value.append(self.represent_mapping(u'tag:yaml.org,2002:map',
+        #        [(item_key, item_value)]))
+        #return SequenceNode(u'tag:yaml.org,2002:pairs', value)
+
+    def represent_dict(self, data):
+        return self.represent_mapping(u'tag:yaml.org,2002:map', data)
+
+    def represent_set(self, data):
+        value = {}
+        for key in data:
+            value[key] = None
+        return self.represent_mapping(u'tag:yaml.org,2002:set', value)
+
+    def represent_date(self, data):
+        value = unicode(data.isoformat())
+        return self.represent_scalar(u'tag:yaml.org,2002:timestamp', value)
+
+    def represent_datetime(self, data):
+        value = unicode(data.isoformat(' '))
+        return self.represent_scalar(u'tag:yaml.org,2002:timestamp', value)
+
+    def represent_yaml_object(self, tag, data, cls, flow_style=None):
+        if hasattr(data, '__getstate__'):
+            state = data.__getstate__()
+        else:
+            state = data.__dict__.copy()
+        return self.represent_mapping(tag, state, flow_style=flow_style)
+
+    def represent_undefined(self, data):
+        raise RepresenterError("cannot represent an object: %s" % data)
+
+SafeRepresenter.add_representer(type(None),
+        SafeRepresenter.represent_none)
+
+SafeRepresenter.add_representer(str,
+        SafeRepresenter.represent_str)
+
+SafeRepresenter.add_representer(unicode,
+        SafeRepresenter.represent_unicode)
+
+SafeRepresenter.add_representer(bool,
+        SafeRepresenter.represent_bool)
+
+SafeRepresenter.add_representer(int,
+        SafeRepresenter.represent_int)
+
+SafeRepresenter.add_representer(long,
+        SafeRepresenter.represent_long)
+
+SafeRepresenter.add_representer(float,
+        SafeRepresenter.represent_float)
+
+SafeRepresenter.add_representer(list,
+        SafeRepresenter.represent_list)
+
+SafeRepresenter.add_representer(tuple,
+        SafeRepresenter.represent_list)
+
+SafeRepresenter.add_representer(dict,
+        SafeRepresenter.represent_dict)
+
+SafeRepresenter.add_representer(set,
+        SafeRepresenter.represent_set)
+
+SafeRepresenter.add_representer(datetime.date,
+        SafeRepresenter.represent_date)
+
+SafeRepresenter.add_representer(datetime.datetime,
+        SafeRepresenter.represent_datetime)
+
+SafeRepresenter.add_representer(None,
+        SafeRepresenter.represent_undefined)
+
+class Representer(SafeRepresenter):
+
+    def represent_str(self, data):
+        tag = None
+        style = None
+        try:
+            data = unicode(data, 'ascii')
+            tag = u'tag:yaml.org,2002:str'
+        except UnicodeDecodeError:
+            try:
+                data = unicode(data, 'utf-8')
+                tag = u'tag:yaml.org,2002:python/str'
+            except UnicodeDecodeError:
+                data = data.encode('base64')
+                tag = u'tag:yaml.org,2002:binary'
+                style = '|'
+        return self.represent_scalar(tag, data, style=style)
+
+    def represent_unicode(self, data):
+        tag = None
+        try:
+            data.encode('ascii')
+            tag = u'tag:yaml.org,2002:python/unicode'
+        except UnicodeEncodeError:
+            tag = u'tag:yaml.org,2002:str'
+        return self.represent_scalar(tag, data)
+
+    def represent_long(self, data):
+        tag = u'tag:yaml.org,2002:int'
+        if int(data) is not data:
+            tag = u'tag:yaml.org,2002:python/long'
+        return self.represent_scalar(tag, unicode(data))
+
+    def represent_complex(self, data):
+        if data.imag == 0.0:
+            data = u'%r' % data.real
+        elif data.real == 0.0:
+            data = u'%rj' % data.imag
+        elif data.imag > 0:
+            data = u'%r+%rj' % (data.real, data.imag)
+        else:
+            data = u'%r%rj' % (data.real, data.imag)
+        return self.represent_scalar(u'tag:yaml.org,2002:python/complex', data)
+
+    def represent_tuple(self, data):
+        return self.represent_sequence(u'tag:yaml.org,2002:python/tuple', data)
+
+    def represent_name(self, data):
+        name = u'%s.%s' % (data.__module__, data.__name__)
+        return self.represent_scalar(u'tag:yaml.org,2002:python/name:'+name, u'')
+
+    def represent_module(self, data):
+        return self.represent_scalar(
+                u'tag:yaml.org,2002:python/module:'+data.__name__, u'')
+
+    def represent_instance(self, data):
+        # For instances of classic classes, we use __getinitargs__ and
+        # __getstate__ to serialize the data.
+
+        # If data.__getinitargs__ exists, the object must be reconstructed by
+        # calling cls(**args), where args is a tuple returned by
+        # __getinitargs__. Otherwise, the cls.__init__ method should never be
+        # called and the class instance is created by instantiating a trivial
+        # class and assigning to the instance's __class__ variable.
+
+        # If data.__getstate__ exists, it returns the state of the object.
+        # Otherwise, the state of the object is data.__dict__.
+
+        # We produce either a !!python/object or !!python/object/new node.
+        # If data.__getinitargs__ does not exist and state is a dictionary, we
+        # produce a !!python/object node . Otherwise we produce a
+        # !!python/object/new node.
+
+        cls = data.__class__
+        class_name = u'%s.%s' % (cls.__module__, cls.__name__)
+        args = None
+        state = None
+        if hasattr(data, '__getinitargs__'):
+            args = list(data.__getinitargs__())
+        if hasattr(data, '__getstate__'):
+            state = data.__getstate__()
+        else:
+            state = data.__dict__
+        if args is None and isinstance(state, dict):
+            return self.represent_mapping(
+                    u'tag:yaml.org,2002:python/object:'+class_name, state)
+        if isinstance(state, dict) and not state:
+            return self.represent_sequence(
+                    u'tag:yaml.org,2002:python/object/new:'+class_name, args)
+        value = {}
+        if args:
+            value['args'] = args
+        value['state'] = state
+        return self.represent_mapping(
+                u'tag:yaml.org,2002:python/object/new:'+class_name, value)
+
+    def represent_object(self, data):
+        # We use __reduce__ API to save the data. data.__reduce__ returns
+        # a tuple of length 2-5:
+        #   (function, args, state, listitems, dictitems)
+
+        # For reconstructing, we calls function(*args), then set its state,
+        # listitems, and dictitems if they are not None.
+
+        # A special case is when function.__name__ == '__newobj__'. In this
+        # case we create the object with args[0].__new__(*args).
+
+        # Another special case is when __reduce__ returns a string - we don't
+        # support it.
+
+        # We produce a !!python/object, !!python/object/new or
+        # !!python/object/apply node.
+
+        cls = type(data)
+        if cls in copy_reg.dispatch_table:
+            reduce = copy_reg.dispatch_table[cls](data)
+        elif hasattr(data, '__reduce_ex__'):
+            reduce = data.__reduce_ex__(2)
+        elif hasattr(data, '__reduce__'):
+            reduce = data.__reduce__()
+        else:
+            raise RepresenterError("cannot represent object: %r" % data)
+        reduce = (list(reduce)+[None]*5)[:5]
+        function, args, state, listitems, dictitems = reduce
+        args = list(args)
+        if state is None:
+            state = {}
+        if listitems is not None:
+            listitems = list(listitems)
+        if dictitems is not None:
+            dictitems = dict(dictitems)
+        if function.__name__ == '__newobj__':
+            function = args[0]
+            args = args[1:]
+            tag = u'tag:yaml.org,2002:python/object/new:'
+            newobj = True
+        else:
+            tag = u'tag:yaml.org,2002:python/object/apply:'
+            newobj = False
+        function_name = u'%s.%s' % (function.__module__, function.__name__)
+        if not args and not listitems and not dictitems \
+                and isinstance(state, dict) and newobj:
+            return self.represent_mapping(
+                    u'tag:yaml.org,2002:python/object:'+function_name, state)
+        if not listitems and not dictitems  \
+                and isinstance(state, dict) and not state:
+            return self.represent_sequence(tag+function_name, args)
+        value = {}
+        if args:
+            value['args'] = args
+        if state or not isinstance(state, dict):
+            value['state'] = state
+        if listitems:
+            value['listitems'] = listitems
+        if dictitems:
+            value['dictitems'] = dictitems
+        return self.represent_mapping(tag+function_name, value)
+
+Representer.add_representer(str,
+        Representer.represent_str)
+
+Representer.add_representer(unicode,
+        Representer.represent_unicode)
+
+Representer.add_representer(long,
+        Representer.represent_long)
+
+Representer.add_representer(complex,
+        Representer.represent_complex)
+
+Representer.add_representer(tuple,
+        Representer.represent_tuple)
+
+Representer.add_representer(type,
+        Representer.represent_name)
+
+Representer.add_representer(types.ClassType,
+        Representer.represent_name)
+
+Representer.add_representer(types.FunctionType,
+        Representer.represent_name)
+
+Representer.add_representer(types.BuiltinFunctionType,
+        Representer.represent_name)
+
+Representer.add_representer(types.ModuleType,
+        Representer.represent_module)
+
+Representer.add_multi_representer(types.InstanceType,
+        Representer.represent_instance)
+
+Representer.add_multi_representer(object,
+        Representer.represent_object)
+
diff --git a/lib/spack/external/yaml/resolver.py b/lib/spack/external/yaml/resolver.py
new file mode 100644
index 0000000000..6b5ab87596
--- /dev/null
+++ b/lib/spack/external/yaml/resolver.py
@@ -0,0 +1,224 @@
+
+__all__ = ['BaseResolver', 'Resolver']
+
+from error import *
+from nodes import *
+
+import re
+
+class ResolverError(YAMLError):
+    pass
+
+class BaseResolver(object):
+
+    DEFAULT_SCALAR_TAG = u'tag:yaml.org,2002:str'
+    DEFAULT_SEQUENCE_TAG = u'tag:yaml.org,2002:seq'
+    DEFAULT_MAPPING_TAG = u'tag:yaml.org,2002:map'
+
+    yaml_implicit_resolvers = {}
+    yaml_path_resolvers = {}
+
+    def __init__(self):
+        self.resolver_exact_paths = []
+        self.resolver_prefix_paths = []
+
+    def add_implicit_resolver(cls, tag, regexp, first):
+        if not 'yaml_implicit_resolvers' in cls.__dict__:
+            cls.yaml_implicit_resolvers = cls.yaml_implicit_resolvers.copy()
+        if first is None:
+            first = [None]
+        for ch in first:
+            cls.yaml_implicit_resolvers.setdefault(ch, []).append((tag, regexp))
+    add_implicit_resolver = classmethod(add_implicit_resolver)
+
+    def add_path_resolver(cls, tag, path, kind=None):
+        # Note: `add_path_resolver` is experimental.  The API could be changed.
+        # `new_path` is a pattern that is matched against the path from the
+        # root to the node that is being considered.  `node_path` elements are
+        # tuples `(node_check, index_check)`.  `node_check` is a node class:
+        # `ScalarNode`, `SequenceNode`, `MappingNode` or `None`.  `None`
+        # matches any kind of a node.  `index_check` could be `None`, a boolean
+        # value, a string value, or a number.  `None` and `False` match against
+        # any _value_ of sequence and mapping nodes.  `True` matches against
+        # any _key_ of a mapping node.  A string `index_check` matches against
+        # a mapping value that corresponds to a scalar key which content is
+        # equal to the `index_check` value.  An integer `index_check` matches
+        # against a sequence value with the index equal to `index_check`.
+        if not 'yaml_path_resolvers' in cls.__dict__:
+            cls.yaml_path_resolvers = cls.yaml_path_resolvers.copy()
+        new_path = []
+        for element in path:
+            if isinstance(element, (list, tuple)):
+                if len(element) == 2:
+                    node_check, index_check = element
+                elif len(element) == 1:
+                    node_check = element[0]
+                    index_check = True
+                else:
+                    raise ResolverError("Invalid path element: %s" % element)
+            else:
+                node_check = None
+                index_check = element
+            if node_check is str:
+                node_check = ScalarNode
+            elif node_check is list:
+                node_check = SequenceNode
+            elif node_check is dict:
+                node_check = MappingNode
+            elif node_check not in [ScalarNode, SequenceNode, MappingNode]  \
+                    and not isinstance(node_check, basestring)  \
+                    and node_check is not None:
+                raise ResolverError("Invalid node checker: %s" % node_check)
+            if not isinstance(index_check, (basestring, int))   \
+                    and index_check is not None:
+                raise ResolverError("Invalid index checker: %s" % index_check)
+            new_path.append((node_check, index_check))
+        if kind is str:
+            kind = ScalarNode
+        elif kind is list:
+            kind = SequenceNode
+        elif kind is dict:
+            kind = MappingNode
+        elif kind not in [ScalarNode, SequenceNode, MappingNode]    \
+                and kind is not None:
+            raise ResolverError("Invalid node kind: %s" % kind)
+        cls.yaml_path_resolvers[tuple(new_path), kind] = tag
+    add_path_resolver = classmethod(add_path_resolver)
+
+    def descend_resolver(self, current_node, current_index):
+        if not self.yaml_path_resolvers:
+            return
+        exact_paths = {}
+        prefix_paths = []
+        if current_node:
+            depth = len(self.resolver_prefix_paths)
+            for path, kind in self.resolver_prefix_paths[-1]:
+                if self.check_resolver_prefix(depth, path, kind,
+                        current_node, current_index):
+                    if len(path) > depth:
+                        prefix_paths.append((path, kind))
+                    else:
+                        exact_paths[kind] = self.yaml_path_resolvers[path, kind]
+        else:
+            for path, kind in self.yaml_path_resolvers:
+                if not path:
+                    exact_paths[kind] = self.yaml_path_resolvers[path, kind]
+                else:
+                    prefix_paths.append((path, kind))
+        self.resolver_exact_paths.append(exact_paths)
+        self.resolver_prefix_paths.append(prefix_paths)
+
+    def ascend_resolver(self):
+        if not self.yaml_path_resolvers:
+            return
+        self.resolver_exact_paths.pop()
+        self.resolver_prefix_paths.pop()
+
+    def check_resolver_prefix(self, depth, path, kind,
+            current_node, current_index):
+        node_check, index_check = path[depth-1]
+        if isinstance(node_check, basestring):
+            if current_node.tag != node_check:
+                return
+        elif node_check is not None:
+            if not isinstance(current_node, node_check):
+                return
+        if index_check is True and current_index is not None:
+            return
+        if (index_check is False or index_check is None)    \
+                and current_index is None:
+            return
+        if isinstance(index_check, basestring):
+            if not (isinstance(current_index, ScalarNode)
+                    and index_check == current_index.value):
+                return
+        elif isinstance(index_check, int) and not isinstance(index_check, bool):
+            if index_check != current_index:
+                return
+        return True
+
+    def resolve(self, kind, value, implicit):
+        if kind is ScalarNode and implicit[0]:
+            if value == u'':
+                resolvers = self.yaml_implicit_resolvers.get(u'', [])
+            else:
+                resolvers = self.yaml_implicit_resolvers.get(value[0], [])
+            resolvers += self.yaml_implicit_resolvers.get(None, [])
+            for tag, regexp in resolvers:
+                if regexp.match(value):
+                    return tag
+            implicit = implicit[1]
+        if self.yaml_path_resolvers:
+            exact_paths = self.resolver_exact_paths[-1]
+            if kind in exact_paths:
+                return exact_paths[kind]
+            if None in exact_paths:
+                return exact_paths[None]
+        if kind is ScalarNode:
+            return self.DEFAULT_SCALAR_TAG
+        elif kind is SequenceNode:
+            return self.DEFAULT_SEQUENCE_TAG
+        elif kind is MappingNode:
+            return self.DEFAULT_MAPPING_TAG
+
+class Resolver(BaseResolver):
+    pass
+
+Resolver.add_implicit_resolver(
+        u'tag:yaml.org,2002:bool',
+        re.compile(ur'''^(?:yes|Yes|YES|no|No|NO
+                    |true|True|TRUE|false|False|FALSE
+                    |on|On|ON|off|Off|OFF)$''', re.X),
+        list(u'yYnNtTfFoO'))
+
+Resolver.add_implicit_resolver(
+        u'tag:yaml.org,2002:float',
+        re.compile(ur'''^(?:[-+]?(?:[0-9][0-9_]*)\.[0-9_]*(?:[eE][-+][0-9]+)?
+                    |\.[0-9_]+(?:[eE][-+][0-9]+)?
+                    |[-+]?[0-9][0-9_]*(?::[0-5]?[0-9])+\.[0-9_]*
+                    |[-+]?\.(?:inf|Inf|INF)
+                    |\.(?:nan|NaN|NAN))$''', re.X),
+        list(u'-+0123456789.'))
+
+Resolver.add_implicit_resolver(
+        u'tag:yaml.org,2002:int',
+        re.compile(ur'''^(?:[-+]?0b[0-1_]+
+                    |[-+]?0[0-7_]+
+                    |[-+]?(?:0|[1-9][0-9_]*)
+                    |[-+]?0x[0-9a-fA-F_]+
+                    |[-+]?[1-9][0-9_]*(?::[0-5]?[0-9])+)$''', re.X),
+        list(u'-+0123456789'))
+
+Resolver.add_implicit_resolver(
+        u'tag:yaml.org,2002:merge',
+        re.compile(ur'^(?:<<)$'),
+        [u'<'])
+
+Resolver.add_implicit_resolver(
+        u'tag:yaml.org,2002:null',
+        re.compile(ur'''^(?: ~
+                    |null|Null|NULL
+                    | )$''', re.X),
+        [u'~', u'n', u'N', u''])
+
+Resolver.add_implicit_resolver(
+        u'tag:yaml.org,2002:timestamp',
+        re.compile(ur'''^(?:[0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]
+                    |[0-9][0-9][0-9][0-9] -[0-9][0-9]? -[0-9][0-9]?
+                     (?:[Tt]|[ \t]+)[0-9][0-9]?
+                     :[0-9][0-9] :[0-9][0-9] (?:\.[0-9]*)?
+                     (?:[ \t]*(?:Z|[-+][0-9][0-9]?(?::[0-9][0-9])?))?)$''', re.X),
+        list(u'0123456789'))
+
+Resolver.add_implicit_resolver(
+        u'tag:yaml.org,2002:value',
+        re.compile(ur'^(?:=)$'),
+        [u'='])
+
+# The following resolver is only for documentation purposes. It cannot work
+# because plain scalars cannot start with '!', '&', or '*'.
+Resolver.add_implicit_resolver(
+        u'tag:yaml.org,2002:yaml',
+        re.compile(ur'^(?:!|&|\*)$'),
+        list(u'!&*'))
+
diff --git a/lib/spack/external/yaml/scanner.py b/lib/spack/external/yaml/scanner.py
new file mode 100644
index 0000000000..5228fad65c
--- /dev/null
+++ b/lib/spack/external/yaml/scanner.py
@@ -0,0 +1,1457 @@
+
+# Scanner produces tokens of the following types:
+# STREAM-START
+# STREAM-END
+# DIRECTIVE(name, value)
+# DOCUMENT-START
+# DOCUMENT-END
+# BLOCK-SEQUENCE-START
+# BLOCK-MAPPING-START
+# BLOCK-END
+# FLOW-SEQUENCE-START
+# FLOW-MAPPING-START
+# FLOW-SEQUENCE-END
+# FLOW-MAPPING-END
+# BLOCK-ENTRY
+# FLOW-ENTRY
+# KEY
+# VALUE
+# ALIAS(value)
+# ANCHOR(value)
+# TAG(value)
+# SCALAR(value, plain, style)
+#
+# Read comments in the Scanner code for more details.
+#
+
+__all__ = ['Scanner', 'ScannerError']
+
+from error import MarkedYAMLError
+from tokens import *
+
+class ScannerError(MarkedYAMLError):
+    pass
+
+class SimpleKey(object):
+    # See below simple keys treatment.
+
+    def __init__(self, token_number, required, index, line, column, mark):
+        self.token_number = token_number
+        self.required = required
+        self.index = index
+        self.line = line
+        self.column = column
+        self.mark = mark
+
+class Scanner(object):
+
+    def __init__(self):
+        """Initialize the scanner."""
+        # It is assumed that Scanner and Reader will have a common descendant.
+        # Reader do the dirty work of checking for BOM and converting the
+        # input data to Unicode. It also adds NUL to the end.
+        #
+        # Reader supports the following methods
+        #   self.peek(i=0)       # peek the next i-th character
+        #   self.prefix(l=1)     # peek the next l characters
+        #   self.forward(l=1)    # read the next l characters and move the pointer.
+
+        # Had we reached the end of the stream?
+        self.done = False
+
+        # The number of unclosed '{' and '['. `flow_level == 0` means block
+        # context.
+        self.flow_level = 0
+
+        # List of processed tokens that are not yet emitted.
+        self.tokens = []
+
+        # Add the STREAM-START token.
+        self.fetch_stream_start()
+
+        # Number of tokens that were emitted through the `get_token` method.
+        self.tokens_taken = 0
+
+        # The current indentation level.
+        self.indent = -1
+
+        # Past indentation levels.
+        self.indents = []
+
+        # Variables related to simple keys treatment.
+
+        # A simple key is a key that is not denoted by the '?' indicator.
+        # Example of simple keys:
+        #   ---
+        #   block simple key: value
+        #   ? not a simple key:
+        #   : { flow simple key: value }
+        # We emit the KEY token before all keys, so when we find a potential
+        # simple key, we try to locate the corresponding ':' indicator.
+        # Simple keys should be limited to a single line and 1024 characters.
+
+        # Can a simple key start at the current position? A simple key may
+        # start:
+        # - at the beginning of the line, not counting indentation spaces
+        #       (in block context),
+        # - after '{', '[', ',' (in the flow context),
+        # - after '?', ':', '-' (in the block context).
+        # In the block context, this flag also signifies if a block collection
+        # may start at the current position.
+        self.allow_simple_key = True
+
+        # Keep track of possible simple keys. This is a dictionary. The key
+        # is `flow_level`; there can be no more that one possible simple key
+        # for each level. The value is a SimpleKey record:
+        #   (token_number, required, index, line, column, mark)
+        # A simple key may start with ALIAS, ANCHOR, TAG, SCALAR(flow),
+        # '[', or '{' tokens.
+        self.possible_simple_keys = {}
+
+    # Public methods.
+
+    def check_token(self, *choices):
+        # Check if the next token is one of the given types.
+        while self.need_more_tokens():
+            self.fetch_more_tokens()
+        if self.tokens:
+            if not choices:
+                return True
+            for choice in choices:
+                if isinstance(self.tokens[0], choice):
+                    return True
+        return False
+
+    def peek_token(self):
+        # Return the next token, but do not delete if from the queue.
+        while self.need_more_tokens():
+            self.fetch_more_tokens()
+        if self.tokens:
+            return self.tokens[0]
+
+    def get_token(self):
+        # Return the next token.
+        while self.need_more_tokens():
+            self.fetch_more_tokens()
+        if self.tokens:
+            self.tokens_taken += 1
+            return self.tokens.pop(0)
+
+    # Private methods.
+
+    def need_more_tokens(self):
+        if self.done:
+            return False
+        if not self.tokens:
+            return True
+        # The current token may be a potential simple key, so we
+        # need to look further.
+        self.stale_possible_simple_keys()
+        if self.next_possible_simple_key() == self.tokens_taken:
+            return True
+
+    def fetch_more_tokens(self):
+
+        # Eat whitespaces and comments until we reach the next token.
+        self.scan_to_next_token()
+
+        # Remove obsolete possible simple keys.
+        self.stale_possible_simple_keys()
+
+        # Compare the current indentation and column. It may add some tokens
+        # and decrease the current indentation level.
+        self.unwind_indent(self.column)
+
+        # Peek the next character.
+        ch = self.peek()
+
+        # Is it the end of stream?
+        if ch == u'\0':
+            return self.fetch_stream_end()
+
+        # Is it a directive?
+        if ch == u'%' and self.check_directive():
+            return self.fetch_directive()
+
+        # Is it the document start?
+        if ch == u'-' and self.check_document_start():
+            return self.fetch_document_start()
+
+        # Is it the document end?
+        if ch == u'.' and self.check_document_end():
+            return self.fetch_document_end()
+
+        # TODO: support for BOM within a stream.
+        #if ch == u'\uFEFF':
+        #    return self.fetch_bom()    <-- issue BOMToken
+
+        # Note: the order of the following checks is NOT significant.
+
+        # Is it the flow sequence start indicator?
+        if ch == u'[':
+            return self.fetch_flow_sequence_start()
+
+        # Is it the flow mapping start indicator?
+        if ch == u'{':
+            return self.fetch_flow_mapping_start()
+
+        # Is it the flow sequence end indicator?
+        if ch == u']':
+            return self.fetch_flow_sequence_end()
+
+        # Is it the flow mapping end indicator?
+        if ch == u'}':
+            return self.fetch_flow_mapping_end()
+
+        # Is it the flow entry indicator?
+        if ch == u',':
+            return self.fetch_flow_entry()
+
+        # Is it the block entry indicator?
+        if ch == u'-' and self.check_block_entry():
+            return self.fetch_block_entry()
+
+        # Is it the key indicator?
+        if ch == u'?' and self.check_key():
+            return self.fetch_key()
+
+        # Is it the value indicator?
+        if ch == u':' and self.check_value():
+            return self.fetch_value()
+
+        # Is it an alias?
+        if ch == u'*':
+            return self.fetch_alias()
+
+        # Is it an anchor?
+        if ch == u'&':
+            return self.fetch_anchor()
+
+        # Is it a tag?
+        if ch == u'!':
+            return self.fetch_tag()
+
+        # Is it a literal scalar?
+        if ch == u'|' and not self.flow_level:
+            return self.fetch_literal()
+
+        # Is it a folded scalar?
+        if ch == u'>' and not self.flow_level:
+            return self.fetch_folded()
+
+        # Is it a single quoted scalar?
+        if ch == u'\'':
+            return self.fetch_single()
+
+        # Is it a double quoted scalar?
+        if ch == u'\"':
+            return self.fetch_double()
+
+        # It must be a plain scalar then.
+        if self.check_plain():
+            return self.fetch_plain()
+
+        # No? It's an error. Let's produce a nice error message.
+        raise ScannerError("while scanning for the next token", None,
+                "found character %r that cannot start any token"
+                % ch.encode('utf-8'), self.get_mark())
+
+    # Simple keys treatment.
+
+    def next_possible_simple_key(self):
+        # Return the number of the nearest possible simple key. Actually we
+        # don't need to loop through the whole dictionary. We may replace it
+        # with the following code:
+        #   if not self.possible_simple_keys:
+        #       return None
+        #   return self.possible_simple_keys[
+        #           min(self.possible_simple_keys.keys())].token_number
+        min_token_number = None
+        for level in self.possible_simple_keys:
+            key = self.possible_simple_keys[level]
+            if min_token_number is None or key.token_number < min_token_number:
+                min_token_number = key.token_number
+        return min_token_number
+
+    def stale_possible_simple_keys(self):
+        # Remove entries that are no longer possible simple keys. According to
+        # the YAML specification, simple keys
+        # - should be limited to a single line,
+        # - should be no longer than 1024 characters.
+        # Disabling this procedure will allow simple keys of any length and
+        # height (may cause problems if indentation is broken though).
+        for level in self.possible_simple_keys.keys():
+            key = self.possible_simple_keys[level]
+            if key.line != self.line  \
+                    or self.index-key.index > 1024:
+                if key.required:
+                    raise ScannerError("while scanning a simple key", key.mark,
+                            "could not found expected ':'", self.get_mark())
+                del self.possible_simple_keys[level]
+
+    def save_possible_simple_key(self):
+        # The next token may start a simple key. We check if it's possible
+        # and save its position. This function is called for
+        #   ALIAS, ANCHOR, TAG, SCALAR(flow), '[', and '{'.
+
+        # Check if a simple key is required at the current position.
+        required = not self.flow_level and self.indent == self.column
+
+        # A simple key is required only if it is the first token in the current
+        # line. Therefore it is always allowed.
+        assert self.allow_simple_key or not required
+
+        # The next token might be a simple key. Let's save it's number and
+        # position.
+        if self.allow_simple_key:
+            self.remove_possible_simple_key()
+            token_number = self.tokens_taken+len(self.tokens)
+            key = SimpleKey(token_number, required,
+                    self.index, self.line, self.column, self.get_mark())
+            self.possible_simple_keys[self.flow_level] = key
+
+    def remove_possible_simple_key(self):
+        # Remove the saved possible key position at the current flow level.
+        if self.flow_level in self.possible_simple_keys:
+            key = self.possible_simple_keys[self.flow_level]
+            
+            if key.required:
+                raise ScannerError("while scanning a simple key", key.mark,
+                        "could not found expected ':'", self.get_mark())
+
+            del self.possible_simple_keys[self.flow_level]
+
+    # Indentation functions.
+
+    def unwind_indent(self, column):
+
+        ## In flow context, tokens should respect indentation.
+        ## Actually the condition should be `self.indent >= column` according to
+        ## the spec. But this condition will prohibit intuitively correct
+        ## constructions such as
+        ## key : {
+        ## }
+        #if self.flow_level and self.indent > column:
+        #    raise ScannerError(None, None,
+        #            "invalid intendation or unclosed '[' or '{'",
+        #            self.get_mark())
+
+        # In the flow context, indentation is ignored. We make the scanner less
+        # restrictive then specification requires.
+        if self.flow_level:
+            return
+
+        # In block context, we may need to issue the BLOCK-END tokens.
+        while self.indent > column:
+            mark = self.get_mark()
+            self.indent = self.indents.pop()
+            self.tokens.append(BlockEndToken(mark, mark))
+
+    def add_indent(self, column):
+        # Check if we need to increase indentation.
+        if self.indent < column:
+            self.indents.append(self.indent)
+            self.indent = column
+            return True
+        return False
+
+    # Fetchers.
+
+    def fetch_stream_start(self):
+        # We always add STREAM-START as the first token and STREAM-END as the
+        # last token.
+
+        # Read the token.
+        mark = self.get_mark()
+        
+        # Add STREAM-START.
+        self.tokens.append(StreamStartToken(mark, mark,
+            encoding=self.encoding))
+        
+
+    def fetch_stream_end(self):
+
+        # Set the current intendation to -1.
+        self.unwind_indent(-1)
+
+        # Reset simple keys.
+        self.remove_possible_simple_key()
+        self.allow_simple_key = False
+        self.possible_simple_keys = {}
+
+        # Read the token.
+        mark = self.get_mark()
+        
+        # Add STREAM-END.
+        self.tokens.append(StreamEndToken(mark, mark))
+
+        # The steam is finished.
+        self.done = True
+
+    def fetch_directive(self):
+        
+        # Set the current intendation to -1.
+        self.unwind_indent(-1)
+
+        # Reset simple keys.
+        self.remove_possible_simple_key()
+        self.allow_simple_key = False
+
+        # Scan and add DIRECTIVE.
+        self.tokens.append(self.scan_directive())
+
+    def fetch_document_start(self):
+        self.fetch_document_indicator(DocumentStartToken)
+
+    def fetch_document_end(self):
+        self.fetch_document_indicator(DocumentEndToken)
+
+    def fetch_document_indicator(self, TokenClass):
+
+        # Set the current intendation to -1.
+        self.unwind_indent(-1)
+
+        # Reset simple keys. Note that there could not be a block collection
+        # after '---'.
+        self.remove_possible_simple_key()
+        self.allow_simple_key = False
+
+        # Add DOCUMENT-START or DOCUMENT-END.
+        start_mark = self.get_mark()
+        self.forward(3)
+        end_mark = self.get_mark()
+        self.tokens.append(TokenClass(start_mark, end_mark))
+
+    def fetch_flow_sequence_start(self):
+        self.fetch_flow_collection_start(FlowSequenceStartToken)
+
+    def fetch_flow_mapping_start(self):
+        self.fetch_flow_collection_start(FlowMappingStartToken)
+
+    def fetch_flow_collection_start(self, TokenClass):
+
+        # '[' and '{' may start a simple key.
+        self.save_possible_simple_key()
+
+        # Increase the flow level.
+        self.flow_level += 1
+
+        # Simple keys are allowed after '[' and '{'.
+        self.allow_simple_key = True
+
+        # Add FLOW-SEQUENCE-START or FLOW-MAPPING-START.
+        start_mark = self.get_mark()
+        self.forward()
+        end_mark = self.get_mark()
+        self.tokens.append(TokenClass(start_mark, end_mark))
+
+    def fetch_flow_sequence_end(self):
+        self.fetch_flow_collection_end(FlowSequenceEndToken)
+
+    def fetch_flow_mapping_end(self):
+        self.fetch_flow_collection_end(FlowMappingEndToken)
+
+    def fetch_flow_collection_end(self, TokenClass):
+
+        # Reset possible simple key on the current level.
+        self.remove_possible_simple_key()
+
+        # Decrease the flow level.
+        self.flow_level -= 1
+
+        # No simple keys after ']' or '}'.
+        self.allow_simple_key = False
+
+        # Add FLOW-SEQUENCE-END or FLOW-MAPPING-END.
+        start_mark = self.get_mark()
+        self.forward()
+        end_mark = self.get_mark()
+        self.tokens.append(TokenClass(start_mark, end_mark))
+
+    def fetch_flow_entry(self):
+
+        # Simple keys are allowed after ','.
+        self.allow_simple_key = True
+
+        # Reset possible simple key on the current level.
+        self.remove_possible_simple_key()
+
+        # Add FLOW-ENTRY.
+        start_mark = self.get_mark()
+        self.forward()
+        end_mark = self.get_mark()
+        self.tokens.append(FlowEntryToken(start_mark, end_mark))
+
+    def fetch_block_entry(self):
+
+        # Block context needs additional checks.
+        if not self.flow_level:
+
+            # Are we allowed to start a new entry?
+            if not self.allow_simple_key:
+                raise ScannerError(None, None,
+                        "sequence entries are not allowed here",
+                        self.get_mark())
+
+            # We may need to add BLOCK-SEQUENCE-START.
+            if self.add_indent(self.column):
+                mark = self.get_mark()
+                self.tokens.append(BlockSequenceStartToken(mark, mark))
+
+        # It's an error for the block entry to occur in the flow context,
+        # but we let the parser detect this.
+        else:
+            pass
+
+        # Simple keys are allowed after '-'.
+        self.allow_simple_key = True
+
+        # Reset possible simple key on the current level.
+        self.remove_possible_simple_key()
+
+        # Add BLOCK-ENTRY.
+        start_mark = self.get_mark()
+        self.forward()
+        end_mark = self.get_mark()
+        self.tokens.append(BlockEntryToken(start_mark, end_mark))
+
+    def fetch_key(self):
+        
+        # Block context needs additional checks.
+        if not self.flow_level:
+
+            # Are we allowed to start a key (not nessesary a simple)?
+            if not self.allow_simple_key:
+                raise ScannerError(None, None,
+                        "mapping keys are not allowed here",
+                        self.get_mark())
+
+            # We may need to add BLOCK-MAPPING-START.
+            if self.add_indent(self.column):
+                mark = self.get_mark()
+                self.tokens.append(BlockMappingStartToken(mark, mark))
+
+        # Simple keys are allowed after '?' in the block context.
+        self.allow_simple_key = not self.flow_level
+
+        # Reset possible simple key on the current level.
+        self.remove_possible_simple_key()
+
+        # Add KEY.
+        start_mark = self.get_mark()
+        self.forward()
+        end_mark = self.get_mark()
+        self.tokens.append(KeyToken(start_mark, end_mark))
+
+    def fetch_value(self):
+
+        # Do we determine a simple key?
+        if self.flow_level in self.possible_simple_keys:
+
+            # Add KEY.
+            key = self.possible_simple_keys[self.flow_level]
+            del self.possible_simple_keys[self.flow_level]
+            self.tokens.insert(key.token_number-self.tokens_taken,
+                    KeyToken(key.mark, key.mark))
+
+            # If this key starts a new block mapping, we need to add
+            # BLOCK-MAPPING-START.
+            if not self.flow_level:
+                if self.add_indent(key.column):
+                    self.tokens.insert(key.token_number-self.tokens_taken,
+                            BlockMappingStartToken(key.mark, key.mark))
+
+            # There cannot be two simple keys one after another.
+            self.allow_simple_key = False
+
+        # It must be a part of a complex key.
+        else:
+            
+            # Block context needs additional checks.
+            # (Do we really need them? They will be catched by the parser
+            # anyway.)
+            if not self.flow_level:
+
+                # We are allowed to start a complex value if and only if
+                # we can start a simple key.
+                if not self.allow_simple_key:
+                    raise ScannerError(None, None,
+                            "mapping values are not allowed here",
+                            self.get_mark())
+
+            # If this value starts a new block mapping, we need to add
+            # BLOCK-MAPPING-START.  It will be detected as an error later by
+            # the parser.
+            if not self.flow_level:
+                if self.add_indent(self.column):
+                    mark = self.get_mark()
+                    self.tokens.append(BlockMappingStartToken(mark, mark))
+
+            # Simple keys are allowed after ':' in the block context.
+            self.allow_simple_key = not self.flow_level
+
+            # Reset possible simple key on the current level.
+            self.remove_possible_simple_key()
+
+        # Add VALUE.
+        start_mark = self.get_mark()
+        self.forward()
+        end_mark = self.get_mark()
+        self.tokens.append(ValueToken(start_mark, end_mark))
+
+    def fetch_alias(self):
+
+        # ALIAS could be a simple key.
+        self.save_possible_simple_key()
+
+        # No simple keys after ALIAS.
+        self.allow_simple_key = False
+
+        # Scan and add ALIAS.
+        self.tokens.append(self.scan_anchor(AliasToken))
+
+    def fetch_anchor(self):
+
+        # ANCHOR could start a simple key.
+        self.save_possible_simple_key()
+
+        # No simple keys after ANCHOR.
+        self.allow_simple_key = False
+
+        # Scan and add ANCHOR.
+        self.tokens.append(self.scan_anchor(AnchorToken))
+
+    def fetch_tag(self):
+
+        # TAG could start a simple key.
+        self.save_possible_simple_key()
+
+        # No simple keys after TAG.
+        self.allow_simple_key = False
+
+        # Scan and add TAG.
+        self.tokens.append(self.scan_tag())
+
+    def fetch_literal(self):
+        self.fetch_block_scalar(style='|')
+
+    def fetch_folded(self):
+        self.fetch_block_scalar(style='>')
+
+    def fetch_block_scalar(self, style):
+
+        # A simple key may follow a block scalar.
+        self.allow_simple_key = True
+
+        # Reset possible simple key on the current level.
+        self.remove_possible_simple_key()
+
+        # Scan and add SCALAR.
+        self.tokens.append(self.scan_block_scalar(style))
+
+    def fetch_single(self):
+        self.fetch_flow_scalar(style='\'')
+
+    def fetch_double(self):
+        self.fetch_flow_scalar(style='"')
+
+    def fetch_flow_scalar(self, style):
+
+        # A flow scalar could be a simple key.
+        self.save_possible_simple_key()
+
+        # No simple keys after flow scalars.
+        self.allow_simple_key = False
+
+        # Scan and add SCALAR.
+        self.tokens.append(self.scan_flow_scalar(style))
+
+    def fetch_plain(self):
+
+        # A plain scalar could be a simple key.
+        self.save_possible_simple_key()
+
+        # No simple keys after plain scalars. But note that `scan_plain` will
+        # change this flag if the scan is finished at the beginning of the
+        # line.
+        self.allow_simple_key = False
+
+        # Scan and add SCALAR. May change `allow_simple_key`.
+        self.tokens.append(self.scan_plain())
+
+    # Checkers.
+
+    def check_directive(self):
+
+        # DIRECTIVE:        ^ '%' ...
+        # The '%' indicator is already checked.
+        if self.column == 0:
+            return True
+
+    def check_document_start(self):
+
+        # DOCUMENT-START:   ^ '---' (' '|'\n')
+        if self.column == 0:
+            if self.prefix(3) == u'---'  \
+                    and self.peek(3) in u'\0 \t\r\n\x85\u2028\u2029':
+                return True
+
+    def check_document_end(self):
+
+        # DOCUMENT-END:     ^ '...' (' '|'\n')
+        if self.column == 0:
+            if self.prefix(3) == u'...'  \
+                    and self.peek(3) in u'\0 \t\r\n\x85\u2028\u2029':
+                return True
+
+    def check_block_entry(self):
+
+        # BLOCK-ENTRY:      '-' (' '|'\n')
+        return self.peek(1) in u'\0 \t\r\n\x85\u2028\u2029'
+
+    def check_key(self):
+
+        # KEY(flow context):    '?'
+        if self.flow_level:
+            return True
+
+        # KEY(block context):   '?' (' '|'\n')
+        else:
+            return self.peek(1) in u'\0 \t\r\n\x85\u2028\u2029'
+
+    def check_value(self):
+
+        # VALUE(flow context):  ':'
+        if self.flow_level:
+            return True
+
+        # VALUE(block context): ':' (' '|'\n')
+        else:
+            return self.peek(1) in u'\0 \t\r\n\x85\u2028\u2029'
+
+    def check_plain(self):
+
+        # A plain scalar may start with any non-space character except:
+        #   '-', '?', ':', ',', '[', ']', '{', '}',
+        #   '#', '&', '*', '!', '|', '>', '\'', '\"',
+        #   '%', '@', '`'.
+        #
+        # It may also start with
+        #   '-', '?', ':'
+        # if it is followed by a non-space character.
+        #
+        # Note that we limit the last rule to the block context (except the
+        # '-' character) because we want the flow context to be space
+        # independent.
+        ch = self.peek()
+        return ch not in u'\0 \t\r\n\x85\u2028\u2029-?:,[]{}#&*!|>\'\"%@`'  \
+                or (self.peek(1) not in u'\0 \t\r\n\x85\u2028\u2029'
+                        and (ch == u'-' or (not self.flow_level and ch in u'?:')))
+
+    # Scanners.
+
+    def scan_to_next_token(self):
+        # We ignore spaces, line breaks and comments.
+        # If we find a line break in the block context, we set the flag
+        # `allow_simple_key` on.
+        # The byte order mark is stripped if it's the first character in the
+        # stream. We do not yet support BOM inside the stream as the
+        # specification requires. Any such mark will be considered as a part
+        # of the document.
+        #
+        # TODO: We need to make tab handling rules more sane. A good rule is
+        #   Tabs cannot precede tokens
+        #   BLOCK-SEQUENCE-START, BLOCK-MAPPING-START, BLOCK-END,
+        #   KEY(block), VALUE(block), BLOCK-ENTRY
+        # So the checking code is
+        #   if <TAB>:
+        #       self.allow_simple_keys = False
+        # We also need to add the check for `allow_simple_keys == True` to
+        # `unwind_indent` before issuing BLOCK-END.
+        # Scanners for block, flow, and plain scalars need to be modified.
+
+        if self.index == 0 and self.peek() == u'\uFEFF':
+            self.forward()
+        found = False
+        while not found:
+            while self.peek() == u' ':
+                self.forward()
+            if self.peek() == u'#':
+                while self.peek() not in u'\0\r\n\x85\u2028\u2029':
+                    self.forward()
+            if self.scan_line_break():
+                if not self.flow_level:
+                    self.allow_simple_key = True
+            else:
+                found = True
+
+    def scan_directive(self):
+        # See the specification for details.
+        start_mark = self.get_mark()
+        self.forward()
+        name = self.scan_directive_name(start_mark)
+        value = None
+        if name == u'YAML':
+            value = self.scan_yaml_directive_value(start_mark)
+            end_mark = self.get_mark()
+        elif name == u'TAG':
+            value = self.scan_tag_directive_value(start_mark)
+            end_mark = self.get_mark()
+        else:
+            end_mark = self.get_mark()
+            while self.peek() not in u'\0\r\n\x85\u2028\u2029':
+                self.forward()
+        self.scan_directive_ignored_line(start_mark)
+        return DirectiveToken(name, value, start_mark, end_mark)
+
+    def scan_directive_name(self, start_mark):
+        # See the specification for details.
+        length = 0
+        ch = self.peek(length)
+        while u'0' <= ch <= u'9' or u'A' <= ch <= u'Z' or u'a' <= ch <= u'z'    \
+                or ch in u'-_':
+            length += 1
+            ch = self.peek(length)
+        if not length:
+            raise ScannerError("while scanning a directive", start_mark,
+                    "expected alphabetic or numeric character, but found %r"
+                    % ch.encode('utf-8'), self.get_mark())
+        value = self.prefix(length)
+        self.forward(length)
+        ch = self.peek()
+        if ch not in u'\0 \r\n\x85\u2028\u2029':
+            raise ScannerError("while scanning a directive", start_mark,
+                    "expected alphabetic or numeric character, but found %r"
+                    % ch.encode('utf-8'), self.get_mark())
+        return value
+
+    def scan_yaml_directive_value(self, start_mark):
+        # See the specification for details.
+        while self.peek() == u' ':
+            self.forward()
+        major = self.scan_yaml_directive_number(start_mark)
+        if self.peek() != '.':
+            raise ScannerError("while scanning a directive", start_mark,
+                    "expected a digit or '.', but found %r"
+                    % self.peek().encode('utf-8'),
+                    self.get_mark())
+        self.forward()
+        minor = self.scan_yaml_directive_number(start_mark)
+        if self.peek() not in u'\0 \r\n\x85\u2028\u2029':
+            raise ScannerError("while scanning a directive", start_mark,
+                    "expected a digit or ' ', but found %r"
+                    % self.peek().encode('utf-8'),
+                    self.get_mark())
+        return (major, minor)
+
+    def scan_yaml_directive_number(self, start_mark):
+        # See the specification for details.
+        ch = self.peek()
+        if not (u'0' <= ch <= u'9'):
+            raise ScannerError("while scanning a directive", start_mark,
+                    "expected a digit, but found %r" % ch.encode('utf-8'),
+                    self.get_mark())
+        length = 0
+        while u'0' <= self.peek(length) <= u'9':
+            length += 1
+        value = int(self.prefix(length))
+        self.forward(length)
+        return value
+
+    def scan_tag_directive_value(self, start_mark):
+        # See the specification for details.
+        while self.peek() == u' ':
+            self.forward()
+        handle = self.scan_tag_directive_handle(start_mark)
+        while self.peek() == u' ':
+            self.forward()
+        prefix = self.scan_tag_directive_prefix(start_mark)
+        return (handle, prefix)
+
+    def scan_tag_directive_handle(self, start_mark):
+        # See the specification for details.
+        value = self.scan_tag_handle('directive', start_mark)
+        ch = self.peek()
+        if ch != u' ':
+            raise ScannerError("while scanning a directive", start_mark,
+                    "expected ' ', but found %r" % ch.encode('utf-8'),
+                    self.get_mark())
+        return value
+
+    def scan_tag_directive_prefix(self, start_mark):
+        # See the specification for details.
+        value = self.scan_tag_uri('directive', start_mark)
+        ch = self.peek()
+        if ch not in u'\0 \r\n\x85\u2028\u2029':
+            raise ScannerError("while scanning a directive", start_mark,
+                    "expected ' ', but found %r" % ch.encode('utf-8'),
+                    self.get_mark())
+        return value
+
+    def scan_directive_ignored_line(self, start_mark):
+        # See the specification for details.
+        while self.peek() == u' ':
+            self.forward()
+        if self.peek() == u'#':
+            while self.peek() not in u'\0\r\n\x85\u2028\u2029':
+                self.forward()
+        ch = self.peek()
+        if ch not in u'\0\r\n\x85\u2028\u2029':
+            raise ScannerError("while scanning a directive", start_mark,
+                    "expected a comment or a line break, but found %r"
+                        % ch.encode('utf-8'), self.get_mark())
+        self.scan_line_break()
+
+    def scan_anchor(self, TokenClass):
+        # The specification does not restrict characters for anchors and
+        # aliases. This may lead to problems, for instance, the document:
+        #   [ *alias, value ]
+        # can be interpteted in two ways, as
+        #   [ "value" ]
+        # and
+        #   [ *alias , "value" ]
+        # Therefore we restrict aliases to numbers and ASCII letters.
+        start_mark = self.get_mark()
+        indicator = self.peek()
+        if indicator == u'*':
+            name = 'alias'
+        else:
+            name = 'anchor'
+        self.forward()
+        length = 0
+        ch = self.peek(length)
+        while u'0' <= ch <= u'9' or u'A' <= ch <= u'Z' or u'a' <= ch <= u'z'    \
+                or ch in u'-_':
+            length += 1
+            ch = self.peek(length)
+        if not length:
+            raise ScannerError("while scanning an %s" % name, start_mark,
+                    "expected alphabetic or numeric character, but found %r"
+                    % ch.encode('utf-8'), self.get_mark())
+        value = self.prefix(length)
+        self.forward(length)
+        ch = self.peek()
+        if ch not in u'\0 \t\r\n\x85\u2028\u2029?:,]}%@`':
+            raise ScannerError("while scanning an %s" % name, start_mark,
+                    "expected alphabetic or numeric character, but found %r"
+                    % ch.encode('utf-8'), self.get_mark())
+        end_mark = self.get_mark()
+        return TokenClass(value, start_mark, end_mark)
+
+    def scan_tag(self):
+        # See the specification for details.
+        start_mark = self.get_mark()
+        ch = self.peek(1)
+        if ch == u'<':
+            handle = None
+            self.forward(2)
+            suffix = self.scan_tag_uri('tag', start_mark)
+            if self.peek() != u'>':
+                raise ScannerError("while parsing a tag", start_mark,
+                        "expected '>', but found %r" % self.peek().encode('utf-8'),
+                        self.get_mark())
+            self.forward()
+        elif ch in u'\0 \t\r\n\x85\u2028\u2029':
+            handle = None
+            suffix = u'!'
+            self.forward()
+        else:
+            length = 1
+            use_handle = False
+            while ch not in u'\0 \r\n\x85\u2028\u2029':
+                if ch == u'!':
+                    use_handle = True
+                    break
+                length += 1
+                ch = self.peek(length)
+            handle = u'!'
+            if use_handle:
+                handle = self.scan_tag_handle('tag', start_mark)
+            else:
+                handle = u'!'
+                self.forward()
+            suffix = self.scan_tag_uri('tag', start_mark)
+        ch = self.peek()
+        if ch not in u'\0 \r\n\x85\u2028\u2029':
+            raise ScannerError("while scanning a tag", start_mark,
+                    "expected ' ', but found %r" % ch.encode('utf-8'),
+                    self.get_mark())
+        value = (handle, suffix)
+        end_mark = self.get_mark()
+        return TagToken(value, start_mark, end_mark)
+
+    def scan_block_scalar(self, style):
+        # See the specification for details.
+
+        if style == '>':
+            folded = True
+        else:
+            folded = False
+
+        chunks = []
+        start_mark = self.get_mark()
+
+        # Scan the header.
+        self.forward()
+        chomping, increment = self.scan_block_scalar_indicators(start_mark)
+        self.scan_block_scalar_ignored_line(start_mark)
+
+        # Determine the indentation level and go to the first non-empty line.
+        min_indent = self.indent+1
+        if min_indent < 1:
+            min_indent = 1
+        if increment is None:
+            breaks, max_indent, end_mark = self.scan_block_scalar_indentation()
+            indent = max(min_indent, max_indent)
+        else:
+            indent = min_indent+increment-1
+            breaks, end_mark = self.scan_block_scalar_breaks(indent)
+        line_break = u''
+
+        # Scan the inner part of the block scalar.
+        while self.column == indent and self.peek() != u'\0':
+            chunks.extend(breaks)
+            leading_non_space = self.peek() not in u' \t'
+            length = 0
+            while self.peek(length) not in u'\0\r\n\x85\u2028\u2029':
+                length += 1
+            chunks.append(self.prefix(length))
+            self.forward(length)
+            line_break = self.scan_line_break()
+            breaks, end_mark = self.scan_block_scalar_breaks(indent)
+            if self.column == indent and self.peek() != u'\0':
+
+                # Unfortunately, folding rules are ambiguous.
+                #
+                # This is the folding according to the specification:
+                
+                if folded and line_break == u'\n'   \
+                        and leading_non_space and self.peek() not in u' \t':
+                    if not breaks:
+                        chunks.append(u' ')
+                else:
+                    chunks.append(line_break)
+                
+                # This is Clark Evans's interpretation (also in the spec
+                # examples):
+                #
+                #if folded and line_break == u'\n':
+                #    if not breaks:
+                #        if self.peek() not in ' \t':
+                #            chunks.append(u' ')
+                #        else:
+                #            chunks.append(line_break)
+                #else:
+                #    chunks.append(line_break)
+            else:
+                break
+
+        # Chomp the tail.
+        if chomping is not False:
+            chunks.append(line_break)
+        if chomping is True:
+            chunks.extend(breaks)
+
+        # We are done.
+        return ScalarToken(u''.join(chunks), False, start_mark, end_mark,
+                style)
+
+    def scan_block_scalar_indicators(self, start_mark):
+        # See the specification for details.
+        chomping = None
+        increment = None
+        ch = self.peek()
+        if ch in u'+-':
+            if ch == '+':
+                chomping = True
+            else:
+                chomping = False
+            self.forward()
+            ch = self.peek()
+            if ch in u'0123456789':
+                increment = int(ch)
+                if increment == 0:
+                    raise ScannerError("while scanning a block scalar", start_mark,
+                            "expected indentation indicator in the range 1-9, but found 0",
+                            self.get_mark())
+                self.forward()
+        elif ch in u'0123456789':
+            increment = int(ch)
+            if increment == 0:
+                raise ScannerError("while scanning a block scalar", start_mark,
+                        "expected indentation indicator in the range 1-9, but found 0",
+                        self.get_mark())
+            self.forward()
+            ch = self.peek()
+            if ch in u'+-':
+                if ch == '+':
+                    chomping = True
+                else:
+                    chomping = False
+                self.forward()
+        ch = self.peek()
+        if ch not in u'\0 \r\n\x85\u2028\u2029':
+            raise ScannerError("while scanning a block scalar", start_mark,
+                    "expected chomping or indentation indicators, but found %r"
+                        % ch.encode('utf-8'), self.get_mark())
+        return chomping, increment
+
+    def scan_block_scalar_ignored_line(self, start_mark):
+        # See the specification for details.
+        while self.peek() == u' ':
+            self.forward()
+        if self.peek() == u'#':
+            while self.peek() not in u'\0\r\n\x85\u2028\u2029':
+                self.forward()
+        ch = self.peek()
+        if ch not in u'\0\r\n\x85\u2028\u2029':
+            raise ScannerError("while scanning a block scalar", start_mark,
+                    "expected a comment or a line break, but found %r"
+                        % ch.encode('utf-8'), self.get_mark())
+        self.scan_line_break()
+
+    def scan_block_scalar_indentation(self):
+        # See the specification for details.
+        chunks = []
+        max_indent = 0
+        end_mark = self.get_mark()
+        while self.peek() in u' \r\n\x85\u2028\u2029':
+            if self.peek() != u' ':
+                chunks.append(self.scan_line_break())
+                end_mark = self.get_mark()
+            else:
+                self.forward()
+                if self.column > max_indent:
+                    max_indent = self.column
+        return chunks, max_indent, end_mark
+
+    def scan_block_scalar_breaks(self, indent):
+        # See the specification for details.
+        chunks = []
+        end_mark = self.get_mark()
+        while self.column < indent and self.peek() == u' ':
+            self.forward()
+        while self.peek() in u'\r\n\x85\u2028\u2029':
+            chunks.append(self.scan_line_break())
+            end_mark = self.get_mark()
+            while self.column < indent and self.peek() == u' ':
+                self.forward()
+        return chunks, end_mark
+
+    def scan_flow_scalar(self, style):
+        # See the specification for details.
+        # Note that we loose indentation rules for quoted scalars. Quoted
+        # scalars don't need to adhere indentation because " and ' clearly
+        # mark the beginning and the end of them. Therefore we are less
+        # restrictive then the specification requires. We only need to check
+        # that document separators are not included in scalars.
+        if style == '"':
+            double = True
+        else:
+            double = False
+        chunks = []
+        start_mark = self.get_mark()
+        quote = self.peek()
+        self.forward()
+        chunks.extend(self.scan_flow_scalar_non_spaces(double, start_mark))
+        while self.peek() != quote:
+            chunks.extend(self.scan_flow_scalar_spaces(double, start_mark))
+            chunks.extend(self.scan_flow_scalar_non_spaces(double, start_mark))
+        self.forward()
+        end_mark = self.get_mark()
+        return ScalarToken(u''.join(chunks), False, start_mark, end_mark,
+                style)
+
+    ESCAPE_REPLACEMENTS = {
+        u'0':   u'\0',
+        u'a':   u'\x07',
+        u'b':   u'\x08',
+        u't':   u'\x09',
+        u'\t':  u'\x09',
+        u'n':   u'\x0A',
+        u'v':   u'\x0B',
+        u'f':   u'\x0C',
+        u'r':   u'\x0D',
+        u'e':   u'\x1B',
+        u' ':   u'\x20',
+        u'\"':  u'\"',
+        u'\\':  u'\\',
+        u'N':   u'\x85',
+        u'_':   u'\xA0',
+        u'L':   u'\u2028',
+        u'P':   u'\u2029',
+    }
+
+    ESCAPE_CODES = {
+        u'x':   2,
+        u'u':   4,
+        u'U':   8,
+    }
+
+    def scan_flow_scalar_non_spaces(self, double, start_mark):
+        # See the specification for details.
+        chunks = []
+        while True:
+            length = 0
+            while self.peek(length) not in u'\'\"\\\0 \t\r\n\x85\u2028\u2029':
+                length += 1
+            if length:
+                chunks.append(self.prefix(length))
+                self.forward(length)
+            ch = self.peek()
+            if not double and ch == u'\'' and self.peek(1) == u'\'':
+                chunks.append(u'\'')
+                self.forward(2)
+            elif (double and ch == u'\'') or (not double and ch in u'\"\\'):
+                chunks.append(ch)
+                self.forward()
+            elif double and ch == u'\\':
+                self.forward()
+                ch = self.peek()
+                if ch in self.ESCAPE_REPLACEMENTS:
+                    chunks.append(self.ESCAPE_REPLACEMENTS[ch])
+                    self.forward()
+                elif ch in self.ESCAPE_CODES:
+                    length = self.ESCAPE_CODES[ch]
+                    self.forward()
+                    for k in range(length):
+                        if self.peek(k) not in u'0123456789ABCDEFabcdef':
+                            raise ScannerError("while scanning a double-quoted scalar", start_mark,
+                                    "expected escape sequence of %d hexdecimal numbers, but found %r" %
+                                        (length, self.peek(k).encode('utf-8')), self.get_mark())
+                    code = int(self.prefix(length), 16)
+                    chunks.append(unichr(code))
+                    self.forward(length)
+                elif ch in u'\r\n\x85\u2028\u2029':
+                    self.scan_line_break()
+                    chunks.extend(self.scan_flow_scalar_breaks(double, start_mark))
+                else:
+                    raise ScannerError("while scanning a double-quoted scalar", start_mark,
+                            "found unknown escape character %r" % ch.encode('utf-8'), self.get_mark())
+            else:
+                return chunks
+
+    def scan_flow_scalar_spaces(self, double, start_mark):
+        # See the specification for details.
+        chunks = []
+        length = 0
+        while self.peek(length) in u' \t':
+            length += 1
+        whitespaces = self.prefix(length)
+        self.forward(length)
+        ch = self.peek()
+        if ch == u'\0':
+            raise ScannerError("while scanning a quoted scalar", start_mark,
+                    "found unexpected end of stream", self.get_mark())
+        elif ch in u'\r\n\x85\u2028\u2029':
+            line_break = self.scan_line_break()
+            breaks = self.scan_flow_scalar_breaks(double, start_mark)
+            if line_break != u'\n':
+                chunks.append(line_break)
+            elif not breaks:
+                chunks.append(u' ')
+            chunks.extend(breaks)
+        else:
+            chunks.append(whitespaces)
+        return chunks
+
+    def scan_flow_scalar_breaks(self, double, start_mark):
+        # See the specification for details.
+        chunks = []
+        while True:
+            # Instead of checking indentation, we check for document
+            # separators.
+            prefix = self.prefix(3)
+            if (prefix == u'---' or prefix == u'...')   \
+                    and self.peek(3) in u'\0 \t\r\n\x85\u2028\u2029':
+                raise ScannerError("while scanning a quoted scalar", start_mark,
+                        "found unexpected document separator", self.get_mark())
+            while self.peek() in u' \t':
+                self.forward()
+            if self.peek() in u'\r\n\x85\u2028\u2029':
+                chunks.append(self.scan_line_break())
+            else:
+                return chunks
+
+    def scan_plain(self):
+        # See the specification for details.
+        # We add an additional restriction for the flow context:
+        #   plain scalars in the flow context cannot contain ',', ':' and '?'.
+        # We also keep track of the `allow_simple_key` flag here.
+        # Indentation rules are loosed for the flow context.
+        chunks = []
+        start_mark = self.get_mark()
+        end_mark = start_mark
+        indent = self.indent+1
+        # We allow zero indentation for scalars, but then we need to check for
+        # document separators at the beginning of the line.
+        #if indent == 0:
+        #    indent = 1
+        spaces = []
+        while True:
+            length = 0
+            if self.peek() == u'#':
+                break
+            while True:
+                ch = self.peek(length)
+                if ch in u'\0 \t\r\n\x85\u2028\u2029'   \
+                        or (not self.flow_level and ch == u':' and
+                                self.peek(length+1) in u'\0 \t\r\n\x85\u2028\u2029') \
+                        or (self.flow_level and ch in u',:?[]{}'):
+                    break
+                length += 1
+            # It's not clear what we should do with ':' in the flow context.
+            if (self.flow_level and ch == u':'
+                    and self.peek(length+1) not in u'\0 \t\r\n\x85\u2028\u2029,[]{}'):
+                self.forward(length)
+                raise ScannerError("while scanning a plain scalar", start_mark,
+                    "found unexpected ':'", self.get_mark(),
+                    "Please check http://pyyaml.org/wiki/YAMLColonInFlowContext for details.")
+            if length == 0:
+                break
+            self.allow_simple_key = False
+            chunks.extend(spaces)
+            chunks.append(self.prefix(length))
+            self.forward(length)
+            end_mark = self.get_mark()
+            spaces = self.scan_plain_spaces(indent, start_mark)
+            if not spaces or self.peek() == u'#' \
+                    or (not self.flow_level and self.column < indent):
+                break
+        return ScalarToken(u''.join(chunks), True, start_mark, end_mark)
+
+    def scan_plain_spaces(self, indent, start_mark):
+        # See the specification for details.
+        # The specification is really confusing about tabs in plain scalars.
+        # We just forbid them completely. Do not use tabs in YAML!
+        chunks = []
+        length = 0
+        while self.peek(length) in u' ':
+            length += 1
+        whitespaces = self.prefix(length)
+        self.forward(length)
+        ch = self.peek()
+        if ch in u'\r\n\x85\u2028\u2029':
+            line_break = self.scan_line_break()
+            self.allow_simple_key = True
+            prefix = self.prefix(3)
+            if (prefix == u'---' or prefix == u'...')   \
+                    and self.peek(3) in u'\0 \t\r\n\x85\u2028\u2029':
+                return
+            breaks = []
+            while self.peek() in u' \r\n\x85\u2028\u2029':
+                if self.peek() == ' ':
+                    self.forward()
+                else:
+                    breaks.append(self.scan_line_break())
+                    prefix = self.prefix(3)
+                    if (prefix == u'---' or prefix == u'...')   \
+                            and self.peek(3) in u'\0 \t\r\n\x85\u2028\u2029':
+                        return
+            if line_break != u'\n':
+                chunks.append(line_break)
+            elif not breaks:
+                chunks.append(u' ')
+            chunks.extend(breaks)
+        elif whitespaces:
+            chunks.append(whitespaces)
+        return chunks
+
+    def scan_tag_handle(self, name, start_mark):
+        # See the specification for details.
+        # For some strange reasons, the specification does not allow '_' in
+        # tag handles. I have allowed it anyway.
+        ch = self.peek()
+        if ch != u'!':
+            raise ScannerError("while scanning a %s" % name, start_mark,
+                    "expected '!', but found %r" % ch.encode('utf-8'),
+                    self.get_mark())
+        length = 1
+        ch = self.peek(length)
+        if ch != u' ':
+            while u'0' <= ch <= u'9' or u'A' <= ch <= u'Z' or u'a' <= ch <= u'z'    \
+                    or ch in u'-_':
+                length += 1
+                ch = self.peek(length)
+            if ch != u'!':
+                self.forward(length)
+                raise ScannerError("while scanning a %s" % name, start_mark,
+                        "expected '!', but found %r" % ch.encode('utf-8'),
+                        self.get_mark())
+            length += 1
+        value = self.prefix(length)
+        self.forward(length)
+        return value
+
+    def scan_tag_uri(self, name, start_mark):
+        # See the specification for details.
+        # Note: we do not check if URI is well-formed.
+        chunks = []
+        length = 0
+        ch = self.peek(length)
+        while u'0' <= ch <= u'9' or u'A' <= ch <= u'Z' or u'a' <= ch <= u'z'    \
+                or ch in u'-;/?:@&=+$,_.!~*\'()[]%':
+            if ch == u'%':
+                chunks.append(self.prefix(length))
+                self.forward(length)
+                length = 0
+                chunks.append(self.scan_uri_escapes(name, start_mark))
+            else:
+                length += 1
+            ch = self.peek(length)
+        if length:
+            chunks.append(self.prefix(length))
+            self.forward(length)
+            length = 0
+        if not chunks:
+            raise ScannerError("while parsing a %s" % name, start_mark,
+                    "expected URI, but found %r" % ch.encode('utf-8'),
+                    self.get_mark())
+        return u''.join(chunks)
+
+    def scan_uri_escapes(self, name, start_mark):
+        # See the specification for details.
+        bytes = []
+        mark = self.get_mark()
+        while self.peek() == u'%':
+            self.forward()
+            for k in range(2):
+                if self.peek(k) not in u'0123456789ABCDEFabcdef':
+                    raise ScannerError("while scanning a %s" % name, start_mark,
+                            "expected URI escape sequence of 2 hexdecimal numbers, but found %r" %
+                                (self.peek(k).encode('utf-8')), self.get_mark())
+            bytes.append(chr(int(self.prefix(2), 16)))
+            self.forward(2)
+        try:
+            value = unicode(''.join(bytes), 'utf-8')
+        except UnicodeDecodeError, exc:
+            raise ScannerError("while scanning a %s" % name, start_mark, str(exc), mark)
+        return value
+
+    def scan_line_break(self):
+        # Transforms:
+        #   '\r\n'      :   '\n'
+        #   '\r'        :   '\n'
+        #   '\n'        :   '\n'
+        #   '\x85'      :   '\n'
+        #   '\u2028'    :   '\u2028'
+        #   '\u2029     :   '\u2029'
+        #   default     :   ''
+        ch = self.peek()
+        if ch in u'\r\n\x85':
+            if self.prefix(2) == u'\r\n':
+                self.forward(2)
+            else:
+                self.forward()
+            return u'\n'
+        elif ch in u'\u2028\u2029':
+            self.forward()
+            return ch
+        return u''
+
+#try:
+#    import psyco
+#    psyco.bind(Scanner)
+#except ImportError:
+#    pass
+
diff --git a/lib/spack/external/yaml/serializer.py b/lib/spack/external/yaml/serializer.py
new file mode 100644
index 0000000000..0bf1e96dc1
--- /dev/null
+++ b/lib/spack/external/yaml/serializer.py
@@ -0,0 +1,111 @@
+
+__all__ = ['Serializer', 'SerializerError']
+
+from error import YAMLError
+from events import *
+from nodes import *
+
+class SerializerError(YAMLError):
+    pass
+
+class Serializer(object):
+
+    ANCHOR_TEMPLATE = u'id%03d'
+
+    def __init__(self, encoding=None,
+            explicit_start=None, explicit_end=None, version=None, tags=None):
+        self.use_encoding = encoding
+        self.use_explicit_start = explicit_start
+        self.use_explicit_end = explicit_end
+        self.use_version = version
+        self.use_tags = tags
+        self.serialized_nodes = {}
+        self.anchors = {}
+        self.last_anchor_id = 0
+        self.closed = None
+
+    def open(self):
+        if self.closed is None:
+            self.emit(StreamStartEvent(encoding=self.use_encoding))
+            self.closed = False
+        elif self.closed:
+            raise SerializerError("serializer is closed")
+        else:
+            raise SerializerError("serializer is already opened")
+
+    def close(self):
+        if self.closed is None:
+            raise SerializerError("serializer is not opened")
+        elif not self.closed:
+            self.emit(StreamEndEvent())
+            self.closed = True
+
+    #def __del__(self):
+    #    self.close()
+
+    def serialize(self, node):
+        if self.closed is None:
+            raise SerializerError("serializer is not opened")
+        elif self.closed:
+            raise SerializerError("serializer is closed")
+        self.emit(DocumentStartEvent(explicit=self.use_explicit_start,
+            version=self.use_version, tags=self.use_tags))
+        self.anchor_node(node)
+        self.serialize_node(node, None, None)
+        self.emit(DocumentEndEvent(explicit=self.use_explicit_end))
+        self.serialized_nodes = {}
+        self.anchors = {}
+        self.last_anchor_id = 0
+
+    def anchor_node(self, node):
+        if node in self.anchors:
+            if self.anchors[node] is None:
+                self.anchors[node] = self.generate_anchor(node)
+        else:
+            self.anchors[node] = None
+            if isinstance(node, SequenceNode):
+                for item in node.value:
+                    self.anchor_node(item)
+            elif isinstance(node, MappingNode):
+                for key, value in node.value:
+                    self.anchor_node(key)
+                    self.anchor_node(value)
+
+    def generate_anchor(self, node):
+        self.last_anchor_id += 1
+        return self.ANCHOR_TEMPLATE % self.last_anchor_id
+
+    def serialize_node(self, node, parent, index):
+        alias = self.anchors[node]
+        if node in self.serialized_nodes:
+            self.emit(AliasEvent(alias))
+        else:
+            self.serialized_nodes[node] = True
+            self.descend_resolver(parent, index)
+            if isinstance(node, ScalarNode):
+                detected_tag = self.resolve(ScalarNode, node.value, (True, False))
+                default_tag = self.resolve(ScalarNode, node.value, (False, True))
+                implicit = (node.tag == detected_tag), (node.tag == default_tag)
+                self.emit(ScalarEvent(alias, node.tag, implicit, node.value,
+                    style=node.style))
+            elif isinstance(node, SequenceNode):
+                implicit = (node.tag
+                            == self.resolve(SequenceNode, node.value, True))
+                self.emit(SequenceStartEvent(alias, node.tag, implicit,
+                    flow_style=node.flow_style))
+                index = 0
+                for item in node.value:
+                    self.serialize_node(item, node, index)
+                    index += 1
+                self.emit(SequenceEndEvent())
+            elif isinstance(node, MappingNode):
+                implicit = (node.tag
+                            == self.resolve(MappingNode, node.value, True))
+                self.emit(MappingStartEvent(alias, node.tag, implicit,
+                    flow_style=node.flow_style))
+                for key, value in node.value:
+                    self.serialize_node(key, node, None)
+                    self.serialize_node(value, node, key)
+                self.emit(MappingEndEvent())
+            self.ascend_resolver()
+
diff --git a/lib/spack/external/yaml/tokens.py b/lib/spack/external/yaml/tokens.py
new file mode 100644
index 0000000000..4d0b48a394
--- /dev/null
+++ b/lib/spack/external/yaml/tokens.py
@@ -0,0 +1,104 @@
+
+class Token(object):
+    def __init__(self, start_mark, end_mark):
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+    def __repr__(self):
+        attributes = [key for key in self.__dict__
+                if not key.endswith('_mark')]
+        attributes.sort()
+        arguments = ', '.join(['%s=%r' % (key, getattr(self, key))
+                for key in attributes])
+        return '%s(%s)' % (self.__class__.__name__, arguments)
+
+#class BOMToken(Token):
+#    id = '<byte order mark>'
+
+class DirectiveToken(Token):
+    id = '<directive>'
+    def __init__(self, name, value, start_mark, end_mark):
+        self.name = name
+        self.value = value
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+
+class DocumentStartToken(Token):
+    id = '<document start>'
+
+class DocumentEndToken(Token):
+    id = '<document end>'
+
+class StreamStartToken(Token):
+    id = '<stream start>'
+    def __init__(self, start_mark=None, end_mark=None,
+            encoding=None):
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+        self.encoding = encoding
+
+class StreamEndToken(Token):
+    id = '<stream end>'
+
+class BlockSequenceStartToken(Token):
+    id = '<block sequence start>'
+
+class BlockMappingStartToken(Token):
+    id = '<block mapping start>'
+
+class BlockEndToken(Token):
+    id = '<block end>'
+
+class FlowSequenceStartToken(Token):
+    id = '['
+
+class FlowMappingStartToken(Token):
+    id = '{'
+
+class FlowSequenceEndToken(Token):
+    id = ']'
+
+class FlowMappingEndToken(Token):
+    id = '}'
+
+class KeyToken(Token):
+    id = '?'
+
+class ValueToken(Token):
+    id = ':'
+
+class BlockEntryToken(Token):
+    id = '-'
+
+class FlowEntryToken(Token):
+    id = ','
+
+class AliasToken(Token):
+    id = '<alias>'
+    def __init__(self, value, start_mark, end_mark):
+        self.value = value
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+
+class AnchorToken(Token):
+    id = '<anchor>'
+    def __init__(self, value, start_mark, end_mark):
+        self.value = value
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+
+class TagToken(Token):
+    id = '<tag>'
+    def __init__(self, value, start_mark, end_mark):
+        self.value = value
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+
+class ScalarToken(Token):
+    id = '<scalar>'
+    def __init__(self, value, plain, start_mark, end_mark, style=None):
+        self.value = value
+        self.plain = plain
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+        self.style = style
+
diff --git a/lib/spack/llnl/util/lang.py b/lib/spack/llnl/util/lang.py
index 332367f537..9e1bef18ca 100644
--- a/lib/spack/llnl/util/lang.py
+++ b/lib/spack/llnl/util/lang.py
@@ -126,22 +126,20 @@ def caller_locals():
         del stack
 
 
-def get_calling_package_name():
+def get_calling_module_name():
     """Make sure that the caller is a class definition, and return the
-       module's name.
+       enclosing module's name.
     """
     stack = inspect.stack()
     try:
-        # get calling function name (the relation)
-        relation = stack[1][3]
-
         # Make sure locals contain __module__
         caller_locals = stack[2][0].f_locals
     finally:
         del stack
 
     if not '__module__' in caller_locals:
-        raise ScopeError(relation)
+        raise RuntimeError("Must invoke get_calling_module_name() "
+                           "from inside a class definition!")
 
     module_name = caller_locals['__module__']
     base_name = module_name.split('.')[-1]
@@ -322,6 +320,24 @@ def match_predicate(*args):
     return match
 
 
+
+def DictWrapper(dictionary):
+    """Returns a class that wraps a dictionary and enables it to be used
+       like an object."""
+    class wrapper(object):
+        def __getattr__(self, name):        return dictionary[name]
+        def __setattr__(self, name, value): dictionary[name] = value
+        def setdefault(self, *args):        return dictionary.setdefault(*args)
+        def get(self, *args):               return dictionary.get(*args)
+        def keys(self):                     return dictionary.keys()
+        def values(self):                   return dictionary.values()
+        def items(self):                    return dictionary.items()
+        def __iter__(self):                 return iter(dictionary)
+
+
+    return wrapper()
+
+
 class RequiredAttributeError(ValueError):
     def __init__(self, message):
         super(RequiredAttributeError, self).__init__(message)
diff --git a/lib/spack/spack/__init__.py b/lib/spack/spack/__init__.py
index eb891e3d57..caa09eb6e0 100644
--- a/lib/spack/spack/__init__.py
+++ b/lib/spack/spack/__init__.py
@@ -42,7 +42,8 @@ test_path      = join_path(module_path, "test")
 hooks_path     = join_path(module_path, "hooks")
 var_path       = join_path(prefix, "var", "spack")
 stage_path     = join_path(var_path, "stage")
-install_path   = join_path(prefix, "opt")
+opt_path       = join_path(prefix, "opt")
+install_path   = join_path(opt_path, "spack")
 share_path     = join_path(prefix, "share", "spack")
 
 #
@@ -65,8 +66,8 @@ mock_user_config = join_path(mock_config_path, "user_spackconfig")
 # This controls how spack lays out install prefixes and
 # stage directories.
 #
-from spack.directory_layout import SpecHashDirectoryLayout
-install_layout = SpecHashDirectoryLayout(install_path)
+from spack.directory_layout import YamlDirectoryLayout
+install_layout = YamlDirectoryLayout(install_path)
 
 #
 # This controls how things are concretized in spack.
@@ -146,9 +147,9 @@ import llnl.util.filesystem
 from llnl.util.filesystem import *
 __all__ += llnl.util.filesystem.__all__
 
-import spack.relations
-from spack.relations import *
-__all__ += spack.relations.__all__
+import spack.directives
+from spack.directives import *
+__all__ += spack.directives.__all__
 
 import spack.util.executable
 from spack.util.executable import *
diff --git a/lib/spack/spack/cmd/activate.py b/lib/spack/spack/cmd/activate.py
index 4070baaa70..1004f1f8e6 100644
--- a/lib/spack/spack/cmd/activate.py
+++ b/lib/spack/spack/cmd/activate.py
@@ -38,17 +38,11 @@ def setup_parser(subparser):
 
 
 def activate(parser, args):
-    # TODO: shouldn't have to concretize here.  Fix DAG issues.
-    specs = spack.cmd.parse_specs(args.spec, concretize=True)
+    specs = spack.cmd.parse_specs(args.spec)
     if len(specs) != 1:
         tty.die("activate requires one spec.  %d given." % len(specs))
 
-    # TODO: remove this hack when DAG info is stored in dir layout.
-    # This ensures the ext spec is always normalized properly.
-    spack.db.get(specs[0])
-
     spec = spack.cmd.disambiguate_spec(specs[0])
-
     if not spec.package.is_extension:
         tty.die("%s is not an extension." % spec.name)
 
diff --git a/lib/spack/spack/cmd/compiler.py b/lib/spack/spack/cmd/compiler.py
index e37f44b3b7..2a64dc914e 100644
--- a/lib/spack/spack/cmd/compiler.py
+++ b/lib/spack/spack/cmd/compiler.py
@@ -68,7 +68,7 @@ def compiler_add(args):
         spack.compilers.add_compilers_to_config('user', *compilers)
         n = len(compilers)
         tty.msg("Added %d new compiler%s to %s" % (
-            n, 's' if n > 1 else '', spack.config.get_filename('user')))
+            n, 's' if n > 1 else '', spack.config.get_config_scope_filename('user', 'compilers')))
         colify(reversed(sorted(c.spec for c in compilers)), indent=4)
     else:
         tty.msg("Found no new compilers")
diff --git a/lib/spack/spack/cmd/config.py b/lib/spack/spack/cmd/config.py
index 283bfc19b9..8c18f88b64 100644
--- a/lib/spack/spack/cmd/config.py
+++ b/lib/spack/spack/cmd/config.py
@@ -43,42 +43,27 @@ def setup_parser(subparser):
 
     sp = subparser.add_subparsers(metavar='SUBCOMMAND', dest='config_command')
 
-    set_parser = sp.add_parser('set', help='Set configuration values.')
-    set_parser.add_argument('key', help="Key to set value for.")
-    set_parser.add_argument('value', nargs='?', default=None,
-                            help="Value to associate with key")
-
-    get_parser = sp.add_parser('get', help='Get configuration values.')
-    get_parser.add_argument('key', help="Key to get value for.")
+    get_parser = sp.add_parser('get', help='Print configuration values.')
+    get_parser.add_argument('category', help="Configuration category to print.")
 
     edit_parser = sp.add_parser('edit', help='Edit configuration file.')
-
-
-def config_set(args):
-    # default scope for writing is 'user'
-    if not args.scope:
-        args.scope = 'user'
-
-    config = spack.config.get_config(args.scope)
-    config.set_value(args.key, args.value)
-    config.write()
+    edit_parser.add_argument('category', help="Configuration category to edit")
 
 
 def config_get(args):
-    config = spack.config.get_config(args.scope)
-    print config.get_value(args.key)
+    spack.config.print_category(args.category)
 
 
 def config_edit(args):
     if not args.scope:
         args.scope = 'user'
-    config_file = spack.config.get_filename(args.scope)
+    if not args.category:
+        args.category = None
+    config_file = spack.config.get_config_scope_filename(args.scope, args.category)
     spack.editor(config_file)
 
 
 def config(parser, args):
-    action = { 'set'  : config_set,
-               'get'  : config_get,
+    action = { 'get' : config_get,
                'edit' : config_edit }
     action[args.config_command](args)
-
diff --git a/lib/spack/spack/cmd/deactivate.py b/lib/spack/spack/cmd/deactivate.py
index c9a4d4b2f6..e44be41029 100644
--- a/lib/spack/spack/cmd/deactivate.py
+++ b/lib/spack/spack/cmd/deactivate.py
@@ -44,15 +44,10 @@ def setup_parser(subparser):
 
 
 def deactivate(parser, args):
-    # TODO: shouldn't have to concretize here.  Fix DAG issues.
-    specs = spack.cmd.parse_specs(args.spec, concretize=True)
+    specs = spack.cmd.parse_specs(args.spec)
     if len(specs) != 1:
         tty.die("deactivate requires one spec.  %d given." % len(specs))
 
-    # TODO: remove this hack when DAG info is stored properly.
-    # This ensures the ext spec is always normalized properly.
-    spack.db.get(specs[0])
-
     spec = spack.cmd.disambiguate_spec(specs[0])
     pkg = spec.package
 
@@ -67,9 +62,6 @@ def deactivate(parser, args):
                     ext_pkg.do_deactivate(force=True)
 
         elif pkg.is_extension:
-            # TODO: store DAG info properly (see above)
-            spec.normalize()
-
             if not args.force and not spec.package.activated:
                 tty.die("%s is not activated." % pkg.spec.short_spec)
 
@@ -81,10 +73,6 @@ def deactivate(parser, args):
             for name in topo_order:
                 espec = index[name]
                 epkg = espec.package
-
-                # TODO: store DAG info properly (see above)
-                epkg.spec.normalize()
-
                 if epkg.extends(pkg.extendee_spec):
                     if epkg.activated or args.force:
 
diff --git a/lib/spack/spack/cmd/find.py b/lib/spack/spack/cmd/find.py
index 70b10edb4e..15c1cc9196 100644
--- a/lib/spack/spack/cmd/find.py
+++ b/lib/spack/spack/cmd/find.py
@@ -41,9 +41,6 @@ description ="Find installed spack packages"
 def setup_parser(subparser):
     format_group = subparser.add_mutually_exclusive_group()
     format_group.add_argument(
-        '-l', '--long', action='store_const', dest='mode', const='long',
-        help='Show dependency hashes as well as versions.')
-    format_group.add_argument(
         '-p', '--paths', action='store_const', dest='mode', const='paths',
         help='Show paths to package install directories')
     format_group.add_argument(
@@ -51,12 +48,21 @@ def setup_parser(subparser):
         help='Show full dependency DAG of installed packages')
 
     subparser.add_argument(
+        '-l', '--long', action='store_true', dest='long',
+        help='Show dependency hashes as well as versions.')
+
+    subparser.add_argument(
         'query_specs', nargs=argparse.REMAINDER,
         help='optional specs to filter results')
 
 
+def gray_hash(spec):
+    return colorize('@K{[%s]}' % spec.dag_hash(7))
+
+
 def display_specs(specs, **kwargs):
     mode = kwargs.get('mode', 'short')
+    hashes = kwargs.get('long', False)
 
     # Make a dict with specs keyed by architecture and compiler.
     index = index_by(specs, ('architecture', 'compiler'))
@@ -85,13 +91,20 @@ def display_specs(specs, **kwargs):
 
         elif mode == 'deps':
             for spec in specs:
-                print spec.tree(indent=4, format='$_$@$+$#', color=True),
-
-        elif mode in ('short', 'long'):
-            fmt = '$-_$@$+'
-            if mode == 'long':
-                fmt += '$#'
-            colify(s.format(fmt, color=True) for s in specs)
+                print spec.tree(
+                    format='$_$@$+',
+                    color=True,
+                    indent=4,
+                    prefix=(lambda s: gray_hash(s)) if hashes else None)
+
+        elif mode == 'short':
+            def fmt(s):
+                string = ""
+                if hashes:
+                    string += gray_hash(s) + ' '
+                string += s.format('$-_$@$+', color=True)
+                return string
+            colify(fmt(s) for s in specs)
 
         else:
             raise ValueError(
@@ -125,5 +138,4 @@ def find(parser, args):
 
     if sys.stdout.isatty():
         tty.msg("%d installed packages." % len(specs))
-    display_specs(specs, mode=args.mode)
-
+    display_specs(specs, mode=args.mode, long=args.long)
diff --git a/lib/spack/spack/cmd/info.py b/lib/spack/spack/cmd/info.py
index eafafc601a..c6209523f0 100644
--- a/lib/spack/spack/cmd/info.py
+++ b/lib/spack/spack/cmd/info.py
@@ -22,12 +22,22 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
+import textwrap
 from llnl.util.tty.colify import *
 import spack
 import spack.fetch_strategy as fs
 
 description = "Get detailed information on a particular package"
 
+def padder(str_list, extra=0):
+    """Return a function to pad elements of a list."""
+    length = max(len(str(s)) for s in str_list) + extra
+    def pad(string):
+        string = str(string)
+        padding = max(0, length - len(string))
+        return string + (padding * ' ')
+    return pad
+
 
 def setup_parser(subparser):
     subparser.add_argument('name', metavar="PACKAGE", help="Name of package to get info for.")
@@ -42,13 +52,24 @@ def print_text_info(pkg):
     print "Safe versions:  "
 
     if not pkg.versions:
-        print("None.")
+        print("None")
     else:
-        maxlen = max(len(str(v)) for v in pkg.versions)
-        fmt = "%%-%ss" % maxlen
+        pad = padder(pkg.versions, 4)
         for v in reversed(sorted(pkg.versions)):
             f = fs.for_package_version(pkg, v)
-            print "    " + (fmt % v) + "    " + str(f)
+            print "    %s%s" % (pad(v), str(f))
+
+    print
+    print "Variants:"
+    if not pkg.variants:
+        print "None"
+    else:
+        pad = padder(pkg.variants, 4)
+        for name in sorted(pkg.variants):
+            v = pkg.variants[name]
+            print "    %s%s" % (
+                pad(('+' if v.default else '-') + name + ':'),
+                "\n".join(textwrap.wrap(v.description)))
 
     print
     print "Dependencies:"
diff --git a/lib/spack/spack/cmd/mirror.py b/lib/spack/spack/cmd/mirror.py
index 22838e1344..02a1467ee6 100644
--- a/lib/spack/spack/cmd/mirror.py
+++ b/lib/spack/spack/cmd/mirror.py
@@ -75,27 +75,22 @@ def mirror_add(args):
     if url.startswith('/'):
         url = 'file://' + url
 
-    config = spack.config.get_config('user')
-    config.set_value('mirror', args.name, 'url', url)
-    config.write()
+    mirror_dict = { args.name : url }
+    spack.config.add_to_mirror_config({ args.name : url })
 
 
 def mirror_remove(args):
     """Remove a mirror by name."""
-    config = spack.config.get_config('user')
     name = args.name
 
-    if not config.has_named_section('mirror', name):
+    rmd_something = spack.config.remove_from_config('mirrors', name)
+    if not rmd_something:
         tty.die("No such mirror: %s" % name)
-    config.remove_named_section('mirror', name)
-    config.write()
 
 
 def mirror_list(args):
     """Print out available mirrors to the console."""
-    config = spack.config.get_config()
-    sec_names = config.get_section_names('mirror')
-
+    sec_names = spack.config.get_mirror_config()
     if not sec_names:
         tty.msg("No mirrors configured.")
         return
@@ -103,8 +98,7 @@ def mirror_list(args):
     max_len = max(len(s) for s in sec_names)
     fmt = "%%-%ds%%s" % (max_len + 4)
 
-    for name in sec_names:
-        val = config.get_value('mirror', name, 'url')
+    for name, val in sec_names.iteritems():
         print fmt % (name, val)
 
 
diff --git a/lib/spack/spack/compilers/__init__.py b/lib/spack/spack/compilers/__init__.py
index 8cb11c3208..b7b021a1ac 100644
--- a/lib/spack/spack/compilers/__init__.py
+++ b/lib/spack/spack/compilers/__init__.py
@@ -60,24 +60,25 @@ def _get_config():
        first."""
     # If any configuration file has compilers, just stick with the
     # ones already configured.
-    config = spack.config.get_config()
+    config = spack.config.get_compilers_config()
     existing = [spack.spec.CompilerSpec(s)
-                for s in config.get_section_names('compiler')]
+                for s in config]
     if existing:
         return config
 
     compilers = find_compilers(*get_path('PATH'))
-    new_compilers = [
-        c for c in compilers if c.spec not in existing]
-    add_compilers_to_config('user', *new_compilers)
+    add_compilers_to_config('user', *compilers)
 
     # After writing compilers to the user config, return a full config
     # from all files.
-    return spack.config.get_config(refresh=True)
+    return spack.config.get_compilers_config()
 
 
-@memoized
+_cached_default_compiler = None
 def default_compiler():
+    global _cached_default_compiler
+    if _cached_default_compiler:
+        return _cached_default_compiler
     versions = []
     for name in _default_order:  # TODO: customize order.
         versions = find(name)
@@ -86,7 +87,8 @@ def default_compiler():
     if not versions:
         raise NoCompilersError()
 
-    return sorted(versions)[-1]
+    _cached_default_compiler = sorted(versions)[-1]
+    return _cached_default_compiler
 
 
 def find_compilers(*path):
@@ -122,19 +124,17 @@ def find_compilers(*path):
 
 
 def add_compilers_to_config(scope, *compilers):
-    config = spack.config.get_config(scope)
+    compiler_config_tree = {}
     for compiler in compilers:
-        add_compiler(config, compiler)
-    config.write()
-
-
-def add_compiler(config, compiler):
-    def setup_field(cspec, name, exe):
-        path = exe if exe else "None"
-        config.set_value('compiler', cspec, name, path)
+        compiler_entry = {}
+        for c in _required_instance_vars:
+            val = getattr(compiler, c)
+            if not val:
+                val = "None"
+            compiler_entry[c] = val
+        compiler_config_tree[str(compiler.spec)] = compiler_entry
+    spack.config.add_to_compiler_config(compiler_config_tree, scope)
 
-    for c in _required_instance_vars:
-        setup_field(compiler.spec, c, getattr(compiler, c))
 
 
 def supported_compilers():
@@ -157,8 +157,7 @@ def all_compilers():
        available to build with.  These are instances of CompilerSpec.
     """
     configuration = _get_config()
-    return [spack.spec.CompilerSpec(s)
-            for s in configuration.get_section_names('compiler')]
+    return [spack.spec.CompilerSpec(s) for s in configuration]
 
 
 @_auto_compiler_spec
@@ -176,7 +175,7 @@ def compilers_for_spec(compiler_spec):
     config = _get_config()
 
     def get_compiler(cspec):
-        items = dict((k,v) for k,v in config.items('compiler "%s"' % cspec))
+        items = config[str(cspec)]
 
         if not all(n in items for n in _required_instance_vars):
             raise InvalidCompilerConfigurationError(cspec)
diff --git a/lib/spack/spack/concretize.py b/lib/spack/spack/concretize.py
index 3f569f9dce..15e886ad3c 100644
--- a/lib/spack/spack/concretize.py
+++ b/lib/spack/spack/concretize.py
@@ -101,6 +101,16 @@ class DefaultConcretizer(object):
             spec.architecture = spack.architecture.sys_type()
 
 
+    def concretize_variants(self, spec):
+        """If the spec already has variants filled in, return.  Otherwise, add
+           the default variants from the package specification.
+        """
+        for name, variant in spec.package.variants.items():
+            if name not in spec.variants:
+                spec.variants[name] = spack.spec.VariantSpec(
+                    name, variant.default)
+
+
     def concretize_compiler(self, spec):
         """If the spec already has a compiler, we're done.  If not, then take
            the compiler used for the nearest ancestor with a compiler
diff --git a/lib/spack/spack/config.py b/lib/spack/spack/config.py
index 85ee16a1c2..34dee86473 100644
--- a/lib/spack/spack/config.py
+++ b/lib/spack/spack/config.py
@@ -28,452 +28,315 @@ Configuration file scopes
 ===============================
 
 When Spack runs, it pulls configuration data from several config
-files, much like bash shells.  In Spack, there are two configuration
-scopes:
+directories, each of which contains configuration files.  In Spack,
+there are two configuration scopes:
 
  1. ``site``: Spack loads site-wide configuration options from
-   ``$(prefix)/etc/spackconfig``.
+   ``$(prefix)/etc/spack/``.
 
  2. ``user``: Spack next loads per-user configuration options from
-    ~/.spackconfig.
-
-If user options have the same names as site options, the user options
-take precedence.
+    ~/.spack/.
 
+Spack may read configuration files from both of these locations.  When
+configurations conflict, the user config options take precedence over
+the site configurations.  Each configuration directory may contain
+several configuration files, such as compilers.yaml or mirrors.yaml.
 
 Configuration file format
 ===============================
 
-Configuration files are formatted using .gitconfig syntax, which is
-much like Windows .INI format.  This format is implemented by Python's
-ConfigParser class, and it's easy to read and versatile.
-
-The file is divided into sections, like this ``compiler`` section::
-
-     [compiler]
-         cc = /usr/bin/gcc
-
-In each section there are options (cc), and each option has a value
-(/usr/bin/gcc).
-
-Borrowing from git, we also allow named sections, e.g.:
-
-     [compiler "gcc@4.7.3"]
-         cc = /usr/bin/gcc
-
-This is a compiler section, but it's for the specific compiler,
-``gcc@4.7.3``.  ``gcc@4.7.3`` is the name.
-
-
-Keys
-===============================
-
-Together, the section, name, and option, separated by periods, are
-called a ``key``.  Keys can be used on the command line to set
-configuration options explicitly (this is also borrowed from git).
-
-For example, to change the C compiler used by gcc@4.7.3, you could do
-this:
-
-    spack config compiler.gcc@4.7.3.cc /usr/local/bin/gcc
-
-That will create a named compiler section in the user's .spackconfig
-like the one shown above.
+Configuration files are formatted using YAML syntax.
+This format is implemented by Python's
+yaml class, and it's easy to read and versatile.
+
+The config files are structured as trees, like this ``compiler`` section::
+
+     compilers:
+       chaos_5_x86_64_ib:
+          gcc@4.4.7:
+            cc: /usr/bin/gcc
+            cxx: /usr/bin/g++
+            f77: /usr/bin/gfortran
+            fc: /usr/bin/gfortran
+       bgqos_0:
+          xlc@12.1:
+            cc: /usr/local/bin/mpixlc
+            ...
+
+In this example, entries like ''compilers'' and ''xlc@12.1'' are used to
+categorize entries beneath them in the tree.  At the root of the tree,
+entries like ''cc'' and ''cxx'' are specified as name/value pairs.
+
+Spack returns these trees as nested dicts.  The dict for the above example
+would looks like:
+
+  { 'compilers' :
+      { 'chaos_5_x86_64_ib' :
+         { 'gcc@4.4.7' :
+             { 'cc' : '/usr/bin/gcc',
+               'cxx' : '/usr/bin/g++'
+               'f77' : '/usr/bin/gfortran'
+               'fc' : '/usr/bin/gfortran' }
+         }
+     { 'bgqos_0' :
+         { 'cc' : '/usr/local/bin/mpixlc' }
+     }
+  }
+
+Some routines, like get_mirrors_config and get_compilers_config may strip
+off the top-levels of the tree and return subtrees.
 """
 import os
-import re
-import inspect
-import ConfigParser as cp
+import exceptions
+import sys
 
 from external.ordereddict import OrderedDict
 from llnl.util.lang import memoized
 import spack.error
 
-__all__ = [
-    'SpackConfigParser', 'get_config', 'SpackConfigurationError',
-    'InvalidConfigurationScopeError', 'InvalidSectionNameError',
-    'ReadOnlySpackConfigError', 'ConfigParserError', 'NoOptionError',
-    'NoSectionError']
-
-_named_section_re = r'([^ ]+) "([^"]+)"'
+from contextlib import closing
+from external import yaml
+from external.yaml.error import MarkedYAMLError
+import llnl.util.tty as tty
+from llnl.util.filesystem import mkdirp
+
+_config_sections = {}
+class _ConfigCategory:
+    name = None
+    filename = None
+    merge = True
+    def __init__(self, n, f, m):
+        self.name = n
+        self.filename = f
+        self.merge = m
+        self.files_read_from = []
+        self.result_dict = {}
+        _config_sections[n] = self
+
+_ConfigCategory('compilers', 'compilers.yaml', True)
+_ConfigCategory('mirrors', 'mirrors.yaml', True)
+_ConfigCategory('view', 'views.yaml', True)
+_ConfigCategory('order', 'orders.yaml', True)
 
 """Names of scopes and their corresponding configuration files."""
-_scopes = OrderedDict({
-    'site' : os.path.join(spack.etc_path, 'spackconfig'),
-    'user' : os.path.expanduser('~/.spackconfig')
-})
-
-_field_regex = r'^([\w-]*)'        \
-               r'(?:\.(.*(?=.)))?' \
-               r'(?:\.([\w-]+))?$'
-
-_section_regex = r'^([\w-]*)\s*' \
-                 r'\"([^"]*\)\"$'
-
-
-# Cache of configs -- we memoize this for performance.
-_config = {}
-
-def get_config(scope=None, **kwargs):
-    """Get a Spack configuration object, which can be used to set options.
-
-       With no arguments, this returns a SpackConfigParser with config
-       options loaded from all config files.  This is how client code
-       should read Spack configuration options.
-
-       Optionally, a scope parameter can be provided.  Valid scopes
-       are ``site`` and ``user``.  If a scope is provided, only the
-       options from that scope's configuration file are loaded.  The
-       caller can set or unset options, then call ``write()`` on the
-       config object to write it back out to the original config file.
-
-       By default, this will cache configurations and return the last
-       read version of the config file.  If the config file is
-       modified and you need to refresh, call get_config with the
-       refresh=True keyword argument.  This will force all files to be
-       re-read.
-    """
-    refresh = kwargs.get('refresh', False)
-    if refresh:
-        _config.clear()
-
-    if scope not in _config:
-        if scope is None:
-            _config[scope] = SpackConfigParser([path for path in _scopes.values()])
-        elif scope not in _scopes:
-            raise UnknownConfigurationScopeError(scope)
+config_scopes = [('site', os.path.join(spack.etc_path, 'spack')),
+                 ('user', os.path.expanduser('~/.spack'))]
+
+_compiler_by_arch = {}
+_read_config_file_result = {}
+def _read_config_file(filename):
+    """Read a given YAML configuration file"""
+    global _read_config_file_result
+    if filename in _read_config_file_result:
+        return _read_config_file_result[filename]
+
+    try:
+        with open(filename) as f:
+            ydict = yaml.load(f)
+    except MarkedYAMLError, e:
+        tty.die("Error parsing yaml%s: %s" % (str(e.context_mark), e.problem))
+    except exceptions.IOError, e:
+        _read_config_file_result[filename] = None
+        return None
+    _read_config_file_result[filename] = ydict
+    return ydict
+
+
+def clear_config_caches():
+    """Clears the caches for configuration files, which will cause them
+       to be re-read upon the next request"""
+    for key,s in _config_sections.iteritems():
+        s.files_read_from = []
+        s.result_dict = {}
+    spack.config._read_config_file_result = {}
+    spack.config._compiler_by_arch = {}
+    spack.compilers._cached_default_compiler = None
+
+
+def _merge_dicts(d1, d2):
+    """Recursively merges two configuration trees, with entries
+       in d2 taking precedence over d1"""
+    if not d1:
+        return d2.copy()
+    if not d2:
+        return d1
+
+    for key2, val2 in d2.iteritems():
+        if not key2 in d1:
+            d1[key2] = val2
+            continue
+        val1 = d1[key2]
+        if isinstance(val1, dict) and isinstance(val2, dict):
+            d1[key2] = _merge_dicts(val1, val2)
+            continue
+        if isinstance(val1, list) and isinstance(val2, list):
+            val1.extend(val2)
+            seen = set()
+            d1[key2] = [ x for x in val1 if not (x in seen or seen.add(x)) ]
+            continue
+        d1[key2] = val2
+    return d1
+
+
+def get_config(category_name):
+    """Get the confguration tree for the names category.  Strips off the
+       top-level category entry from the dict"""
+    global config_scopes
+    category = _config_sections[category_name]
+    if category.result_dict:
+        return category.result_dict
+
+    category.result_dict = {}
+    for scope, scope_path in config_scopes:
+        path = os.path.join(scope_path, category.filename)
+        result = _read_config_file(path)
+        if not result:
+            continue
+        if not category_name in result:
+            continue
+        category.files_read_from.insert(0, path)
+        result = result[category_name]
+        if category.merge:
+            category.result_dict = _merge_dicts(category.result_dict, result)
         else:
-            _config[scope] = SpackConfigParser(_scopes[scope])
-
-    return _config[scope]
-
-
-def get_filename(scope):
-    """Get the filename for a particular config scope."""
-    if not scope in _scopes:
-        raise UnknownConfigurationScopeError(scope)
-    return _scopes[scope]
-
-
-def _parse_key(key):
-    """Return the section, name, and option the field describes.
-       Values are returned in a 3-tuple.
-
-       e.g.:
-       The field name ``compiler.gcc@4.7.3.cc`` refers to the 'cc' key
-       in a section that looks like this:
-
-          [compiler "gcc@4.7.3"]
-              cc = /usr/local/bin/gcc
-
-       * The section is ``compiler``
-       * The name is ``gcc@4.7.3``
-       * The key is ``cc``
-    """
-    match = re.search(_field_regex, key)
-    if match:
-        return match.groups()
+            category.result_dict = result
+    return category.result_dict
+
+
+def get_compilers_config(arch=None):
+    """Get the compiler configuration from config files for the given
+       architecture.  Strips off the architecture component of the
+       configuration"""
+    global _compiler_by_arch
+    if not arch:
+        arch = spack.architecture.sys_type()
+    if arch in _compiler_by_arch:
+        return _compiler_by_arch[arch]
+
+    cc_config = get_config('compilers')
+    if arch in cc_config and 'all' in cc_config:
+        arch_compiler = dict(cc_config[arch])
+        _compiler_by_arch[arch] = _merge_dict(arch_compiler, cc_config['all'])
+    elif arch in cc_config:
+        _compiler_by_arch[arch] = cc_config[arch]
+    elif 'all' in cc_config:
+        _compiler_by_arch[arch] = cc_config['all']
     else:
-        raise InvalidSectionNameError(key)
-
-
-def _make_section_name(section, name):
-    if not name:
-        return section
-    return '%s "%s"' % (section, name)
-
-
-def _autokey(fun):
-    """Allow a function to be called with a string key like
-       'compiler.gcc.cc', or with the section, name, and option
-       separated. Function should take at least three args, e.g.:
-
-           fun(self, section, name, option, [...])
-
-       This will allow the function above to be called normally or
-       with a string key, e.g.:
-
-           fun(self, key, [...])
-    """
-    argspec = inspect.getargspec(fun)
-    fun_nargs = len(argspec[0])
-
-    def string_key_func(*args):
-        nargs = len(args)
-        if nargs == fun_nargs - 2:
-            section, name, option = _parse_key(args[1])
-            return fun(args[0], section, name, option, *args[2:])
-
-        elif nargs == fun_nargs:
-            return fun(*args)
-
-        else:
-            raise TypeError(
-                "%s takes %d or %d args (found %d)."
-                % (fun.__name__, fun_nargs - 2, fun_nargs, len(args)))
-    return string_key_func
-
-
-
-class SpackConfigParser(cp.RawConfigParser):
-    """Slightly modified from Python's raw config file parser to accept
-       leading whitespace and preserve comments.
-    """
-    # Slightly modify Python option expressions to allow leading whitespace
-    OPTCRE    = re.compile(r'\s*' + cp.RawConfigParser.OPTCRE.pattern)
-
-    def __init__(self, file_or_files):
-        cp.RawConfigParser.__init__(self, dict_type=OrderedDict)
-
-        if isinstance(file_or_files, basestring):
-            self.read([file_or_files])
-            self.filename = file_or_files
-
-        else:
-            self.read(file_or_files)
-            self.filename = None
-
-
-    @_autokey
-    def set_value(self, section, name, option, value):
-        """Set the value for a key.  If the key is in a section or named
-           section that does not yet exist, add that section.
-        """
-        sn = _make_section_name(section, name)
-        if not self.has_section(sn):
-            self.add_section(sn)
-
-        # Allow valueless config options to be set like this:
-        #     spack config set mirror https://foo.bar.com
-        #
-        # Instead of this, which parses incorrectly:
-        #     spack config set mirror.https://foo.bar.com
-        #
-        if option is None:
-            option = value
-            value = None
-
-        self.set(sn, option, value)
-
-
-    @_autokey
-    def get_value(self, section, name, option):
-        """Get the value for a key.  Raises NoOptionError or NoSectionError if
-           the key is not present."""
-        sn = _make_section_name(section, name)
-
+        _compiler_by_arch[arch] = {}
+    return _compiler_by_arch[arch]
+
+
+def get_mirror_config():
+    """Get the mirror configuration from config files"""
+    return get_config('mirrors')
+
+
+def get_config_scope_dirname(scope):
+    """For a scope return the config directory"""
+    global config_scopes
+    for s,p in config_scopes:
+        if s == scope:
+            return p
+    tty.die("Unknown scope %s.  Valid options are %s" %
+            (scope, ", ".join([s for s,p in config_scopes])))
+
+
+def get_config_scope_filename(scope, category_name):
+    """For some scope and category, get the name of the configuration file"""
+    if not category_name in _config_sections:
+        tty.die("Unknown config category %s.  Valid options are: %s" %
+                (category_name, ", ".join([s for s in _config_sections])))
+    return os.path.join(get_config_scope_dirname(scope), _config_sections[category_name].filename)
+
+
+def add_to_config(category_name, addition_dict, scope=None):
+    """Merge a new dict into a configuration tree and write the new
+       configuration to disk"""
+    global _read_config_file_result
+    get_config(category_name)
+    category = _config_sections[category_name]
+
+    #If scope is specified, use it.  Otherwise use the last config scope that
+    #we successfully parsed data from.
+    file = None
+    path = None
+    if not scope and not category.files_read_from:
+        scope = 'user'
+    if scope:
         try:
-            if not option:
-                # TODO: format this better
-                return self.items(sn)
-
-            return self.get(sn, option)
-
-        # Wrap ConfigParser exceptions in SpackExceptions
-        except cp.NoOptionError, e:  raise NoOptionError(e)
-        except cp.NoSectionError, e: raise NoSectionError(e)
-        except cp.Error, e:          raise ConfigParserError(e)
-
-
-    @_autokey
-    def has_value(self, section, name, option):
-        """Return whether the configuration file has a value for a
-           particular key."""
-        sn = _make_section_name(section, name)
-        return self.has_option(sn, option)
-
-
-    def has_named_section(self, section, name):
-        sn = _make_section_name(section, name)
-        return self.has_section(sn)
-
-
-    def remove_named_section(self, section, name):
-        sn = _make_section_name(section, name)
-        self.remove_section(sn)
-
-
-    def get_section_names(self, sectype):
-        """Get all named sections with the specified type.
-           A named section looks like this:
-
-               [compiler "gcc@4.7"]
-
-           Names of sections are returned as a list, e.g.:
-
-               ['gcc@4.7', 'intel@12.3', 'pgi@4.2']
-
-           You can get items in the sections like this:
-        """
-        sections = []
-        for secname in self.sections():
-            match = re.match(_named_section_re, secname)
-            if match:
-                t, name = match.groups()
-                if t == sectype:
-                    sections.append(name)
-        return sections
-
-
-    def write(self, path_or_fp=None):
-        """Write this configuration out to a file.
-
-           If called with no arguments, this will write the
-           configuration out to the file from which it was read.  If
-           this config was read from multiple files, e.g. site
-           configuration and then user configuration, write will
-           simply raise an error.
-
-           If called with a path or file object, this will write the
-           configuration out to the supplied path or file object.
-        """
-        if path_or_fp is None:
-            if not self.filename:
-                raise ReadOnlySpackConfigError()
-            path_or_fp = self.filename
-
-        if isinstance(path_or_fp, basestring):
-            path_or_fp = open(path_or_fp, 'w')
-
-        self._write(path_or_fp)
-
-
-    def _read(self, fp, fpname):
-        """This is a copy of Python 2.6's _read() method, with support for
-           continuation lines removed."""
-        cursect = None                            # None, or a dictionary
-        optname = None
-        comment = 0
-        lineno = 0
-        e = None                                  # None, or an exception
-        while True:
-            line = fp.readline()
-            if not line:
-                break
-            lineno = lineno + 1
-            # comment or blank line?
-            if ((line.strip() == '' or line[0] in '#;') or
-                (line.split(None, 1)[0].lower() == 'rem' and line[0] in "rR")):
-                self._sections["comment-%d" % comment] = line
-                comment += 1
-            # a section header or option header?
-            else:
-                # is it a section header?
-                mo = self.SECTCRE.match(line)
-                if mo:
-                    sectname = mo.group('header')
-                    if sectname in self._sections:
-                        cursect = self._sections[sectname]
-                    elif sectname == cp.DEFAULTSECT:
-                        cursect = self._defaults
-                    else:
-                        cursect = self._dict()
-                        cursect['__name__'] = sectname
-                        self._sections[sectname] = cursect
-                    # So sections can't start with a continuation line
-                    optname = None
-                # no section header in the file?
-                elif cursect is None:
-                    raise cp.MissingSectionHeaderError(fpname, lineno, line)
-                # an option line?
-                else:
-                    mo = self.OPTCRE.match(line)
-                    if mo:
-                        optname, vi, optval = mo.group('option', 'vi', 'value')
-                        if vi in ('=', ':') and ';' in optval:
-                            # ';' is a comment delimiter only if it follows
-                            # a spacing character
-                            pos = optval.find(';')
-                            if pos != -1 and optval[pos-1].isspace():
-                                optval = optval[:pos]
-                        optval = optval.strip()
-                        # allow empty values
-                        if optval == '""':
-                            optval = ''
-                        optname = self.optionxform(optname.rstrip())
-                        cursect[optname] = optval
-                    else:
-                        # a non-fatal parsing error occurred.  set up the
-                        # exception but keep going. the exception will be
-                        # raised at the end of the file and will contain a
-                        # list of all bogus lines
-                        if not e:
-                            e = cp.ParsingError(fpname)
-                        e.append(lineno, repr(line))
-        # if any parsing errors occurred, raise an exception
-        if e:
-            raise e
-
-
-
-
-    def _write(self, fp):
-        """Write an .ini-format representation of the configuration state.
-
-           This is taken from the default Python 2.6 source.  It writes 4
-           spaces at the beginning of lines instead of no leading space.
-        """
-        if self._defaults:
-            fp.write("[%s]\n" % cp.DEFAULTSECT)
-            for (key, value) in self._defaults.items():
-                fp.write("    %s = %s\n" % (key, str(value).replace('\n', '\n\t')))
-            fp.write("\n")
-
-        for section in self._sections:
-            # Handles comments and blank lines.
-            if isinstance(self._sections[section], basestring):
-                fp.write(self._sections[section])
-                continue
-
-            else:
-                # Allow leading whitespace
-                fp.write("[%s]\n" % section)
-                for (key, value) in self._sections[section].items():
-                    if key != "__name__":
-                        fp.write("    %s = %s\n" %
-                                 (key, str(value).replace('\n', '\n\t')))
-
-
-class SpackConfigurationError(spack.error.SpackError):
-    def __init__(self, *args):
-        super(SpackConfigurationError, self).__init__(*args)
-
-
-class InvalidConfigurationScopeError(SpackConfigurationError):
-    def __init__(self, scope):
-        super(InvalidConfigurationScopeError, self).__init__(
-            "Invalid configuration scope: '%s'" % scope,
-            "Options are: %s" % ", ".join(*_scopes.values()))
-
-
-class InvalidSectionNameError(SpackConfigurationError):
-    """Raised when the name for a section is invalid."""
-    def __init__(self, name):
-        super(InvalidSectionNameError, self).__init__(
-            "Invalid section specifier: '%s'" % name)
-
-
-class ReadOnlySpackConfigError(SpackConfigurationError):
-    """Raised when user attempts to write to a config read from multiple files."""
-    def __init__(self):
-        super(ReadOnlySpackConfigError, self).__init__(
-            "Can only write to a single-file SpackConfigParser")
-
-
-class ConfigParserError(SpackConfigurationError):
-    """Wrapper for the Python ConfigParser's errors"""
-    def __init__(self, error):
-        super(ConfigParserError, self).__init__(str(error))
-        self.error = error
-
-
-class NoOptionError(ConfigParserError):
-    """Wrapper for ConfigParser NoOptionError"""
-    def __init__(self, error):
-        super(NoOptionError, self).__init__(error)
-
-
-class NoSectionError(ConfigParserError):
-    """Wrapper for ConfigParser NoOptionError"""
-    def __init__(self, error):
-        super(NoSectionError, self).__init__(error)
+            dir = get_config_scope_dirname(scope)
+            if not os.path.exists(dir):
+                mkdirp(dir)
+            path = os.path.join(dir, category.filename)
+            file = open(path, 'w')
+        except exceptions.IOError, e:
+            pass
+    else:
+        for p in category.files_read_from:
+            try:
+                file = open(p, 'w')
+            except exceptions.IOError, e:
+                pass
+            if file:
+                path = p
+                break;
+    if not file:
+        tty.die('Unable to write to config file %s' % path)
+
+    #Merge the new information into the existing file info, then write to disk
+    new_dict = _read_config_file_result[path]
+    if new_dict and category_name in new_dict:
+        new_dict = new_dict[category_name]
+    new_dict = _merge_dicts(new_dict, addition_dict)
+    new_dict = { category_name : new_dict }
+    _read_config_file_result[path] = new_dict
+    yaml.dump(new_dict, stream=file, default_flow_style=False)
+    file.close()
+
+    #Merge the new information into the cached results
+    category.result_dict = _merge_dicts(category.result_dict, addition_dict)
+
+
+def add_to_mirror_config(addition_dict, scope=None):
+    """Add mirrors to the configuration files"""
+    add_to_config('mirrors', addition_dict, scope)
+
+
+def add_to_compiler_config(addition_dict, scope=None, arch=None):
+    """Add compilerss to the configuration files"""
+    if not arch:
+        arch = spack.architecture.sys_type()
+    add_to_config('compilers', { arch : addition_dict }, scope)
+    clear_config_caches()
+
+
+def remove_from_config(category_name, key_to_rm, scope=None):
+    """Remove a configuration key and write a new configuration to disk"""
+    global config_scopes
+    get_config(category_name)
+    scopes_to_rm_from = [scope] if scope else [s for s,p in config_scopes]
+    category = _config_sections[category_name]
+
+    rmd_something = False
+    for s in scopes_to_rm_from:
+        path = get_config_scope_filename(scope, category_name)
+        result = _read_config_file(path)
+        if not result:
+            continue
+        if not key_to_rm in result[category_name]:
+            continue
+        with closing(open(path, 'w')) as f:
+            result[category_name].pop(key_to_rm, None)
+            yaml.dump(result, stream=f, default_flow_style=False)
+            category.result_dict.pop(key_to_rm, None)
+            rmd_something = True
+    return rmd_something
+
+
+"""Print a configuration to stdout"""
+def print_category(category_name):
+    if not category_name in _config_sections:
+        tty.die("Unknown config category %s.  Valid options are: %s" %
+                (category_name, ", ".join([s for s in _config_sections])))
+    yaml.dump(get_config(category_name), stream=sys.stdout, default_flow_style=False)
+
diff --git a/lib/spack/spack/directives.py b/lib/spack/spack/directives.py
new file mode 100644
index 0000000000..9297d6dac3
--- /dev/null
+++ b/lib/spack/spack/directives.py
@@ -0,0 +1,277 @@
+##############################################################################
+# Copyright (c) 2013, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://scalability-llnl.github.io/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+"""This package contains directives that can be used within a package.
+
+Directives are functions that can be called inside a package
+definition to modify the package, for example:
+
+    class OpenMpi(Package):
+        depends_on("hwloc")
+        provides("mpi")
+        ...
+
+``provides`` and ``depends_on`` are spack directives.
+
+The available directives are:
+
+  * ``version``
+  * ``depends_on``
+  * ``provides``
+  * ``extends``
+  * ``patch``
+  * ``variant``
+
+"""
+__all__ = [ 'depends_on', 'extends', 'provides', 'patch', 'version',
+            'variant' ]
+
+import re
+import inspect
+
+from llnl.util.lang import *
+
+import spack
+import spack.spec
+import spack.error
+import spack.url
+from spack.version import Version
+from spack.patch import Patch
+from spack.variant import Variant
+from spack.spec import Spec, parse_anonymous_spec
+
+
+#
+# This is a list of all directives, built up as they are defined in
+# this file.
+#
+directives = {}
+
+
+def ensure_dicts(pkg):
+    """Ensure that a package has all the dicts required by directives."""
+    for name, d in directives.items():
+        d.ensure_dicts(pkg)
+
+
+class directive(object):
+    """Decorator for Spack directives.
+
+    Spack directives allow you to modify a package while it is being
+    defined, e.g. to add version or depenency information.  Directives
+    are one of the key pieces of Spack's package "langauge", which is
+    embedded in python.
+
+    Here's an example directive:
+
+        @directive(dicts='versions')
+        version(pkg, ...):
+            ...
+
+    This directive allows you write:
+
+        class Foo(Package):
+            version(...)
+
+    The ``@directive`` decorator handles a couple things for you:
+
+      1. Adds the class scope (pkg) as an initial parameter when
+         called, like a class method would.  This allows you to modify
+         a package from within a directive, while the package is still
+         being defined.
+
+      2. It automatically adds a dictionary called "versions" to the
+         package so that you can refer to pkg.versions.
+
+    The ``(dicts='versions')`` part ensures that ALL packages in Spack
+    will have a ``versions`` attribute after they're constructed, and
+    that if no directive actually modified it, it will just be an
+    empty dict.
+
+    This is just a modular way to add storage attributes to the
+    Package class, and it's how Spack gets information from the
+    packages to the core.
+
+    """
+
+    def __init__(self, dicts=None):
+        if isinstance(dicts, basestring):
+            dicts = (dicts,)
+        elif type(dicts) not in (list, tuple):
+            raise TypeError(
+                "dicts arg must be list, tuple, or string. Found %s."
+                % type(dicts))
+
+        self.dicts = dicts
+
+
+    def ensure_dicts(self, pkg):
+        """Ensure that a package has the dicts required by this directive."""
+        for d in self.dicts:
+            if not hasattr(pkg, d):
+                setattr(pkg, d, {})
+
+            attr = getattr(pkg, d)
+            if not isinstance(attr, dict):
+                raise spack.error.SpackError(
+                    "Package %s has non-dict %s attribute!" % (pkg, d))
+
+
+    def __call__(self, directive_function):
+        directives[directive_function.__name__] = self
+
+        def wrapped(*args, **kwargs):
+            pkg = DictWrapper(caller_locals())
+            self.ensure_dicts(pkg)
+
+            pkg.name = get_calling_module_name()
+            return directive_function(pkg, *args, **kwargs)
+
+        return wrapped
+
+
+@directive('versions')
+def version(pkg, ver, checksum=None, **kwargs):
+    """Adds a version and metadata describing how to fetch it.
+       Metadata is just stored as a dict in the package's versions
+       dictionary.  Package must turn it into a valid fetch strategy
+       later.
+    """
+    # TODO: checksum vs md5 distinction is confusing -- fix this.
+    # special case checksum for backward compatibility
+    if checksum:
+        kwargs['md5'] = checksum
+
+    # Store kwargs for the package to later with a fetch_strategy.
+    pkg.versions[Version(ver)] = kwargs
+
+
+def _depends_on(pkg, spec, when=None):
+    if when is None:
+        when = pkg.name
+    when_spec = parse_anonymous_spec(when, pkg.name)
+
+    dep_spec = Spec(spec)
+    if pkg.name == dep_spec.name:
+        raise CircularReferenceError('depends_on', pkg.name)
+
+    conditions = pkg.dependencies.setdefault(dep_spec.name, {})
+    if when_spec in conditions:
+        conditions[when_spec].constrain(dep_spec, deps=False)
+    else:
+        conditions[when_spec] = dep_spec
+
+
+@directive('dependencies')
+def depends_on(pkg, spec, when=None):
+    """Creates a dict of deps with specs defining when they apply."""
+    _depends_on(pkg, spec, when=when)
+
+
+@directive(('extendees', 'dependencies'))
+def extends(pkg, spec, **kwargs):
+    """Same as depends_on, but dependency is symlinked into parent prefix.
+
+    This is for Python and other language modules where the module
+    needs to be installed into the prefix of the Python installation.
+    Spack handles this by installing modules into their own prefix,
+    but allowing ONE module version to be symlinked into a parent
+    Python install at a time.
+
+    keyword arguments can be passed to extends() so that extension
+    packages can pass parameters to the extendee's extension
+    mechanism.
+
+    """
+    if pkg.extendees:
+        raise DirectiveError("Packages can extend at most one other package.")
+
+    when = kwargs.pop('when', pkg.name)
+    _depends_on(pkg, spec, when=when)
+    pkg.extendees[spec] = (Spec(spec), kwargs)
+
+
+@directive('provided')
+def provides(pkg, *specs, **kwargs):
+    """Allows packages to provide a virtual dependency.  If a package provides
+       'mpi', other packages can declare that they depend on "mpi", and spack
+       can use the providing package to satisfy the dependency.
+    """
+    spec_string = kwargs.get('when', pkg.name)
+    provider_spec = parse_anonymous_spec(spec_string, pkg.name)
+
+    for string in specs:
+        for provided_spec in spack.spec.parse(string):
+            if pkg.name == provided_spec.name:
+                raise CircularReferenceError('depends_on', pkg.name)
+            pkg.provided[provided_spec] = provider_spec
+
+
+@directive('patches')
+def patch(pkg, url_or_filename, level=1, when=None):
+    """Packages can declare patches to apply to source.  You can
+       optionally provide a when spec to indicate that a particular
+       patch should only be applied when the package's spec meets
+       certain conditions (e.g. a particular version).
+    """
+    if when is None:
+        when = pkg.name
+    when_spec = parse_anonymous_spec(when, pkg.name)
+
+    if when_spec not in pkg.patches:
+        pkg.patches[when_spec] = [Patch(pkg.name, url_or_filename, level)]
+    else:
+        # if this spec is identical to some other, then append this
+        # patch to the existing list.
+        pkg.patches[when_spec].append(Patch(pkg.name, url_or_filename, level))
+
+
+@directive('variants')
+def variant(pkg, name, default=False, description=""):
+    """Define a variant for the package. Packager can specify a default
+    value (on or off) as well as a text description."""
+
+    default     = bool(default)
+    description = str(description).strip()
+
+    if not re.match(spack.spec.identifier_re, name):
+        raise DirectiveError("Invalid variant name in %s: '%s'" % (pkg.name, name))
+
+    pkg.variants[name] = Variant(default, description)
+
+
+class DirectiveError(spack.error.SpackError):
+    """This is raised when something is wrong with a package directive."""
+    def __init__(self, directive, message):
+        super(DirectiveError, self).__init__(message)
+        self.directive = directive
+
+
+class CircularReferenceError(DirectiveError):
+    """This is raised when something depends on itself."""
+    def __init__(self, directive, package):
+        super(CircularReferenceError, self).__init__(
+            directive,
+            "Package '%s' cannot pass itself to %s." % (package, directive))
+        self.package = package
diff --git a/lib/spack/spack/directory_layout.py b/lib/spack/spack/directory_layout.py
index b2cf5dc801..fe02fff3b0 100644
--- a/lib/spack/spack/directory_layout.py
+++ b/lib/spack/spack/directory_layout.py
@@ -27,8 +27,9 @@ import os
 import exceptions
 import hashlib
 import shutil
+import glob
 import tempfile
-from contextlib import closing
+from external import yaml
 
 import llnl.util.tty as tty
 from llnl.util.lang import memoized
@@ -81,7 +82,7 @@ class DirectoryLayout(object):
         raise NotImplementedError()
 
 
-    def make_path_for_spec(self, spec):
+    def create_install_directory(self, spec):
         """Creates the installation directory for a spec."""
         raise NotImplementedError()
 
@@ -131,7 +132,7 @@ class DirectoryLayout(object):
         return os.path.join(self.root, path)
 
 
-    def remove_path_for_spec(self, spec):
+    def remove_install_directory(self, spec):
         """Removes a prefix and any empty parent directories from the root.
            Raised RemoveFailedError if something goes wrong.
         """
@@ -153,94 +154,70 @@ class DirectoryLayout(object):
             path = os.path.dirname(path)
 
 
-def traverse_dirs_at_depth(root, depth, path_tuple=(), curdepth=0):
-    """For each directory at <depth> within <root>, return a tuple representing
-       the ancestors of that directory.
-    """
-    if curdepth == depth and curdepth != 0:
-        yield path_tuple
-    elif depth > curdepth:
-        for filename in os.listdir(root):
-            child = os.path.join(root, filename)
-            if os.path.isdir(child):
-                child_tuple = path_tuple + (filename,)
-                for tup in traverse_dirs_at_depth(
-                        child, depth, child_tuple, curdepth+1):
-                    yield tup
-
-
-class SpecHashDirectoryLayout(DirectoryLayout):
+class YamlDirectoryLayout(DirectoryLayout):
     """Lays out installation directories like this::
-           <install_root>/
+           <install root>/
                <architecture>/
-                   <compiler>/
-                       name@version+variant-<dependency_hash>
-
-       Where dependency_hash is a SHA-1 hash prefix for the full package spec.
-       This accounts for dependencies.
+                   <compiler>-<compiler version>/
+                       <name>-<version>-<variants>-<hash>
 
-       If there is ever a hash collision, you won't be able to install a new
-       package unless you use a larger prefix.  However, the full spec is stored
-       in a file called .spec in each directory, so you can migrate an entire
-       install directory to a new hash size pretty easily.
+       The hash here is a SHA-1 hash for the full DAG plus the build
+       spec.  TODO: implement the build spec.
 
-       TODO: make a tool to migrate install directories to different hash sizes.
+       To avoid special characters (like ~) in the directory name,
+       only enabled variants are included in the install path.
+       Disabled variants are omitted.
     """
     def __init__(self, root, **kwargs):
-        """Prefix size is number of characters in the SHA-1 prefix to use
-           to make each hash unique.
-        """
-        spec_file_name = kwargs.get('spec_file_name', '.spec')
-        extension_file_name = kwargs.get('extension_file_name', '.extensions')
-        super(SpecHashDirectoryLayout, self).__init__(root)
-        self.spec_file_name = spec_file_name
-        self.extension_file_name = extension_file_name
+        super(YamlDirectoryLayout, self).__init__(root)
+        self.metadata_dir   = kwargs.get('metadata_dir', '.spack')
+        self.hash_len       = kwargs.get('hash_len', None)
+
+        self.spec_file_name      = 'spec.yaml'
+        self.extension_file_name = 'extensions.yaml'
 
         # Cache of already written/read extension maps.
         self._extension_maps = {}
 
+
     @property
     def hidden_file_paths(self):
-        return ('.spec', '.extensions')
+        return (self.metadata_dir,)
 
 
     def relative_path_for_spec(self, spec):
         _check_concrete(spec)
-        dir_name = spec.format('$_$@$+$#')
-        return join_path(spec.architecture, spec.compiler, dir_name)
+        enabled_variants = (
+            '-' + v.name for v in spec.variants.values()
+            if v.enabled)
+
+        dir_name = "%s-%s%s-%s" % (
+            spec.name,
+            spec.version,
+            ''.join(enabled_variants),
+            spec.dag_hash(self.hash_len))
+
+        path = join_path(
+            spec.architecture,
+            "%s-%s" % (spec.compiler.name, spec.compiler.version),
+            dir_name)
+
+        return path
 
 
     def write_spec(self, spec, path):
         """Write a spec out to a file."""
-        with closing(open(path, 'w')) as spec_file:
-            spec_file.write(spec.tree(ids=False, cover='nodes'))
+        _check_concrete(spec)
+        with open(path, 'w') as f:
+            spec.to_yaml(f)
 
 
     def read_spec(self, path):
         """Read the contents of a file and parse them as a spec"""
-        with closing(open(path)) as spec_file:
-            # Specs from files are assumed normal and concrete
-            spec = Spec(spec_file.read().replace('\n', ''))
-
-        if all(spack.db.exists(s.name) for s in spec.traverse()):
-            copy = spec.copy()
-
-            # TODO: It takes a lot of time to normalize every spec on read.
-            # TODO: Storing graph info with spec files would fix this.
-            copy.normalize()
-            if copy.concrete:
-                return copy   # These are specs spack still understands.
-
-        # If we get here, either the spec is no longer in spack, or
-        # something about its dependencies has changed. So we need to
-        # just assume the read spec is correct.  We'll lose graph
-        # information if we do this, but this is just for best effort
-        # for commands like uninstall and find.  Currently Spack
-        # doesn't do anything that needs the graph info after install.
-
-        # TODO: store specs with full connectivity information, so
-        # that we don't have to normalize or reconstruct based on
-        # changing dependencies in the Spack tree.
+        with open(path) as f:
+            spec = Spec.from_yaml(f)
+
+        # Specs read from actual installations are always concrete
         spec._normal = True
         spec._concrete = True
         return spec
@@ -249,10 +226,14 @@ class SpecHashDirectoryLayout(DirectoryLayout):
     def spec_file_path(self, spec):
         """Gets full path to spec file"""
         _check_concrete(spec)
-        return join_path(self.path_for_spec(spec), self.spec_file_name)
+        return join_path(self.metadata_path(spec), self.spec_file_name)
+
 
+    def metadata_path(self, spec):
+        return join_path(self.path_for_spec(spec), self.metadata_dir)
 
-    def make_path_for_spec(self, spec):
+
+    def create_install_directory(self, spec):
         _check_concrete(spec)
 
         path = self.path_for_spec(spec)
@@ -267,16 +248,13 @@ class SpecHashDirectoryLayout(DirectoryLayout):
             if installed_spec == self.spec:
                 raise InstallDirectoryAlreadyExistsError(path)
 
-            spec_hash = self.hash_spec(spec)
-            installed_hash = self.hash_spec(installed_spec)
-            if installed_spec == spec_hash:
+            if spec.dag_hash() == installed_spec.dag_hash():
                 raise SpecHashCollisionError(installed_hash, spec_hash)
             else:
                 raise InconsistentInstallDirectoryError(
-                    'Spec file in %s does not match SHA-1 hash!'
-                    % spec_file_path)
+                    'Spec file in %s does not match hash!' % spec_file_path)
 
-        mkdirp(path)
+        mkdirp(self.metadata_path(spec))
         self.write_spec(spec, spec_file_path)
 
 
@@ -285,25 +263,50 @@ class SpecHashDirectoryLayout(DirectoryLayout):
         if not os.path.isdir(self.root):
             return []
 
-        specs = []
-        for path in traverse_dirs_at_depth(self.root, 3):
-            arch, compiler, last_dir = path
-            spec_file_path = join_path(
-                self.root, arch, compiler, last_dir, self.spec_file_name)
-            if os.path.exists(spec_file_path):
-                spec = self.read_spec(spec_file_path)
-                specs.append(spec)
-        return specs
+        pattern = join_path(
+            self.root, '*', '*', '*', self.metadata_dir, self.spec_file_name)
+        spec_files = glob.glob(pattern)
+        return [self.read_spec(s) for s in spec_files]
+
+
+    @memoized
+    def specs_by_hash(self):
+        by_hash = {}
+        for spec in self.all_specs():
+            by_hash[spec.dag_hash()] = spec
+        return by_hash
 
 
     def extension_file_path(self, spec):
         """Gets full path to an installed package's extension file"""
         _check_concrete(spec)
-        return join_path(self.path_for_spec(spec), self.extension_file_name)
+        return join_path(self.metadata_path(spec), self.extension_file_name)
+
+
+    def _write_extensions(self, spec, extensions):
+        path = self.extension_file_path(spec)
+
+        # Create a temp file in the same directory as the actual file.
+        dirname, basename = os.path.split(path)
+        tmp = tempfile.NamedTemporaryFile(
+            prefix=basename, dir=dirname, delete=False)
+
+        # write tmp file
+        with tmp:
+            yaml.dump({
+                'extensions' : [
+                    { ext.name : {
+                        'hash' : ext.dag_hash(),
+                        'path' : str(ext.prefix)
+                    }} for ext in sorted(extensions.values())]
+            }, tmp, default_flow_style=False)
+
+        # Atomic update by moving tmpfile on top of old one.
+        os.rename(tmp.name, path)
 
 
     def _extension_map(self, spec):
-        """Get a dict<name -> spec> for all extensions currnetly
+        """Get a dict<name -> spec> for all extensions currently
            installed for this package."""
         _check_concrete(spec)
 
@@ -313,16 +316,26 @@ class SpecHashDirectoryLayout(DirectoryLayout):
                 self._extension_maps[spec] = {}
 
             else:
+                by_hash = self.specs_by_hash()
                 exts = {}
-                with closing(open(path)) as ext_file:
-                    for line in ext_file:
-                        try:
-                            spec = Spec(line.strip())
-                            exts[spec.name] = spec
-                        except spack.error.SpackError, e:
-                            # TODO: do something better here -- should be
-                            # resilient to corrupt files.
-                            raise InvalidExtensionSpecError(str(e))
+                with open(path) as ext_file:
+                    yaml_file = yaml.load(ext_file)
+                    for entry in yaml_file['extensions']:
+                        name = next(iter(entry))
+                        dag_hash = entry[name]['hash']
+                        prefix   = entry[name]['path']
+
+                        if not dag_hash in by_hash:
+                            raise InvalidExtensionSpecError(
+                                "Spec %s not found in %s." % (dag_hash, prefix))
+
+                        ext_spec = by_hash[dag_hash]
+                        if not prefix == ext_spec.prefix:
+                            raise InvalidExtensionSpecError(
+                                "Prefix %s does not match spec with hash %s: %s"
+                                % (prefix, dag_hash, ext_spec))
+
+                        exts[ext_spec.name] = ext_spec
                 self._extension_maps[spec] = exts
 
         return self._extension_maps[spec]
@@ -330,6 +343,7 @@ class SpecHashDirectoryLayout(DirectoryLayout):
 
     def extension_map(self, spec):
         """Defensive copying version of _extension_map() for external API."""
+        _check_concrete(spec)
         return self._extension_map(spec).copy()
 
 
@@ -349,23 +363,6 @@ class SpecHashDirectoryLayout(DirectoryLayout):
             raise NoSuchExtensionError(spec, ext_spec)
 
 
-    def _write_extensions(self, spec, extensions):
-        path = self.extension_file_path(spec)
-
-        # Create a temp file in the same directory as the actual file.
-        dirname, basename = os.path.split(path)
-        tmp = tempfile.NamedTemporaryFile(
-            prefix=basename, dir=dirname, delete=False)
-
-        # Write temp file.
-        with closing(tmp):
-            for extension in sorted(extensions.values()):
-                tmp.write("%s\n" % extension)
-
-        # Atomic update by moving tmpfile on top of old one.
-        os.rename(tmp.name, path)
-
-
     def add_extension(self, spec, ext_spec):
         _check_concrete(spec)
         _check_concrete(ext_spec)
@@ -399,9 +396,9 @@ class DirectoryLayoutError(SpackError):
 
 
 class SpecHashCollisionError(DirectoryLayoutError):
-    """Raised when there is a hash collision in an SpecHashDirectoryLayout."""
+    """Raised when there is a hash collision in an install layout."""
     def __init__(self, installed_spec, new_spec):
-        super(SpecHashDirectoryLayout, self).__init__(
+        super(SpecHashCollisionError, self).__init__(
             'Specs %s and %s have the same SHA-1 prefix!'
             % installed_spec, new_spec)
 
@@ -422,7 +419,7 @@ class InconsistentInstallDirectoryError(DirectoryLayoutError):
 
 
 class InstallDirectoryAlreadyExistsError(DirectoryLayoutError):
-    """Raised when make_path_for_sec is called unnecessarily."""
+    """Raised when create_install_directory is called unnecessarily."""
     def __init__(self, path):
         super(InstallDirectoryAlreadyExistsError, self).__init__(
             "Install path %s already exists!")
@@ -455,5 +452,3 @@ class NoSuchExtensionError(DirectoryLayoutError):
         super(NoSuchExtensionError, self).__init__(
             "%s cannot be removed from %s because it's not activated."% (
                 ext_spec.short_spec, spec.short_spec))
-
-
diff --git a/lib/spack/spack/multimethod.py b/lib/spack/spack/multimethod.py
index 974401e1aa..892619c6ac 100644
--- a/lib/spack/spack/multimethod.py
+++ b/lib/spack/spack/multimethod.py
@@ -195,7 +195,7 @@ class when(object):
     """
 class when(object):
     def __init__(self, spec):
-        pkg = get_calling_package_name()
+        pkg = get_calling_module_name()
         self.spec = parse_anonymous_spec(spec, pkg)
 
     def __call__(self, method):
diff --git a/lib/spack/spack/package.py b/lib/spack/spack/package.py
index 7d9eca5077..452544be49 100644
--- a/lib/spack/spack/package.py
+++ b/lib/spack/spack/package.py
@@ -50,11 +50,11 @@ from llnl.util.filesystem import *
 from llnl.util.lang import *
 
 import spack
-import spack.spec
 import spack.error
 import spack.compilers
 import spack.mirror
 import spack.hooks
+import spack.directives
 import spack.build_environment as build_env
 import spack.url as url
 import spack.fetch_strategy as fs
@@ -301,32 +301,6 @@ class Package(object):
     clean() (some of them do this), and others to provide custom behavior.
 
     """
-
-    #
-    # These variables are defaults for the various "relations".
-    #
-    """Map of information about Versions of this package.
-       Map goes: Version -> dict of attributes"""
-    versions = {}
-
-    """Specs of dependency packages, keyed by name."""
-    dependencies = {}
-
-    """Specs of virtual packages provided by this package, keyed by name."""
-    provided = {}
-
-    """Specs of conflicting packages, keyed by name. """
-    conflicted = {}
-
-    """Patches to apply to newly expanded source, if any."""
-    patches = {}
-
-    """Specs of package this one extends, or None.
-
-    Currently, ppackages can extend at most one other package.
-    """
-    extendees = {}
-
     #
     # These are default values for instance variables.
     #
@@ -350,20 +324,8 @@ class Package(object):
         if '.' in self.name:
             self.name = self.name[self.name.rindex('.') + 1:]
 
-        # Sanity check some required variables that could be
-        # overridden by package authors.
-        def ensure_has_dict(attr_name):
-            if not hasattr(self, attr_name):
-                raise PackageError("Package %s must define %s" % attr_name)
-
-            attr = getattr(self, attr_name)
-            if not isinstance(attr, dict):
-                raise PackageError("Package %s has non-dict %s attribute!"
-                                   % (self.name, attr_name))
-        ensure_has_dict('versions')
-        ensure_has_dict('dependencies')
-        ensure_has_dict('conflicted')
-        ensure_has_dict('patches')
+        # Sanity check attributes required by Spack directives.
+        spack.directives.ensure_dicts(type(self))
 
         # Check versions in the versions dict.
         for v in self.versions:
@@ -577,41 +539,6 @@ class Package(object):
                 yield pkg
 
 
-    def validate_dependencies(self):
-        """Ensure that this package and its dependencies all have consistent
-           constraints on them.
-
-           NOTE that this will NOT find sanity problems through a virtual
-           dependency.  Virtual deps complicate the problem because we
-           don't know in advance which ones conflict with others in the
-           dependency DAG. If there's more than one virtual dependency,
-           it's a full-on SAT problem, so hold off on this for now.
-           The vdeps are actually skipped in preorder_traversal, so see
-           that for details.
-
-           TODO: investigate validating virtual dependencies.
-        """
-        # This algorithm just attempts to merge all the constraints on the same
-        # package together, loses information about the source of the conflict.
-        # What we'd really like to know is exactly which two constraints
-        # conflict, but that algorithm is more expensive, so we'll do it
-        # the simple, less informative way for now.
-        merged = spack.spec.DependencyMap()
-
-        try:
-            for pkg in self.preorder_traversal():
-                for name, spec in pkg.dependencies.iteritems():
-                    if name not in merged:
-                        merged[name] = spec.copy()
-                    else:
-                        merged[name].constrain(spec)
-
-        except spack.spec.UnsatisfiableSpecError, e:
-            raise InvalidPackageDependencyError(
-                "Package %s has inconsistent dependency constraints: %s"
-                % (self.name, e.message))
-
-
     def provides(self, vpkg_name):
         """True if this package provides a virtual package with the specified name."""
         return vpkg_name in self.provided
@@ -664,7 +591,7 @@ class Package(object):
 
     def remove_prefix(self):
         """Removes the prefix for a package along with any empty parent directories."""
-        spack.install_layout.remove_path_for_spec(self.spec)
+        spack.install_layout.remove_install_directory(self.spec)
 
 
     def do_fetch(self):
@@ -820,7 +747,7 @@ class Package(object):
         # create the install directory.  The install layout
         # handles this in case so that it can use whatever
         # package naming scheme it likes.
-        spack.install_layout.make_path_for_spec(self.spec)
+        spack.install_layout.create_install_directory(self.spec)
 
         def cleanup():
             if not keep_prefix:
@@ -841,11 +768,11 @@ class Package(object):
                 spack.hooks.pre_install(self)
 
                 # Set up process's build environment before running install.
-                self.stage.chdir_to_source()
                 if fake_install:
                     self.do_fake_install()
                 else:
                     # Subclasses implement install() to do the real work.
+                    self.stage.chdir_to_source()
                     self.install(self.spec, self.prefix)
 
                 # Ensure that something was actually installed.
@@ -994,16 +921,13 @@ class Package(object):
         self._sanity_check_extension()
         force = kwargs.get('force', False)
 
-        # TODO: get rid of this normalize - DAG handling.
-        self.spec.normalize()
-
-        spack.install_layout.check_extension_conflict(self.extendee_spec, self.spec)
+        spack.install_layout.check_extension_conflict(
+            self.extendee_spec, self.spec)
 
+        # Activate any package dependencies that are also extensions.
         if not force:
             for spec in self.spec.traverse(root=False):
                 if spec.package.extends(self.extendee_spec):
-                    # TODO: fix this normalize() requirement -- revisit DAG handling.
-                    spec.package.spec.normalize()
                     if not spec.package.activated:
                         spec.package.do_activate(**kwargs)
 
@@ -1031,6 +955,7 @@ class Package(object):
         conflict = tree.find_conflict(self.prefix, ignore=ignore)
         if conflict:
             raise ExtensionConflictError(conflict)
+
         tree.merge(self.prefix, ignore=ignore)
 
 
@@ -1237,13 +1162,6 @@ class PackageError(spack.error.SpackError):
         super(PackageError, self).__init__(message, long_msg)
 
 
-class InvalidPackageDependencyError(PackageError):
-    """Raised when package specification is inconsistent with requirements of
-       its dependencies."""
-    def __init__(self, message):
-        super(InvalidPackageDependencyError, self).__init__(message)
-
-
 class PackageVersionError(PackageError):
     """Raised when a version URL cannot automatically be determined."""
     def __init__(self, version):
diff --git a/lib/spack/spack/relations.py b/lib/spack/spack/relations.py
deleted file mode 100644
index a0c7723473..0000000000
--- a/lib/spack/spack/relations.py
+++ /dev/null
@@ -1,215 +0,0 @@
-##############################################################################
-# Copyright (c) 2013, Lawrence Livermore National Security, LLC.
-# Produced at the Lawrence Livermore National Laboratory.
-#
-# This file is part of Spack.
-# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
-# LLNL-CODE-647188
-#
-# For details, see https://scalability-llnl.github.io/spack
-# Please also see the LICENSE file for our notice and the LGPL.
-#
-# This program is free software; you can redistribute it and/or modify
-# it under the terms of the GNU General Public License (as published by
-# the Free Software Foundation) version 2.1 dated February 1999.
-#
-# This program is distributed in the hope that it will be useful, but
-# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
-# conditions of the GNU General Public License for more details.
-#
-# You should have received a copy of the GNU Lesser General Public License
-# along with this program; if not, write to the Free Software Foundation,
-# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
-##############################################################################
-"""
-This package contains relationships that can be defined among packages.
-Relations are functions that can be called inside a package definition,
-for example:
-
-    class OpenMPI(Package):
-        depends_on("hwloc")
-        provides("mpi")
-        ...
-
-The available relations are:
-
-depends_on
-    Above, the OpenMPI package declares that it "depends on" hwloc.  This means
-    that the hwloc package needs to be installed before OpenMPI can be
-    installed.  When a user runs 'spack install openmpi', spack will fetch
-    hwloc and install it first.
-
-provides
-    This is useful when more than one package can satisfy a dependence.  Above,
-    OpenMPI declares that it "provides" mpi.  Other implementations of the MPI
-    interface, like mvapich and mpich, also provide mpi, e.g.:
-
-        class Mvapich(Package):
-            provides("mpi")
-            ...
-
-        class Mpich(Package):
-            provides("mpi")
-            ...
-
-    Instead of depending on openmpi, mvapich, or mpich, another package can
-    declare that it depends on "mpi":
-
-        class Mpileaks(Package):
-            depends_on("mpi")
-            ...
-
-    Now the user can pick which MPI they would like to build with when they
-    install mpileaks.  For example, the user could install 3 instances of
-    mpileaks, one for each MPI version, by issuing these three commands:
-
-        spack install mpileaks ^openmpi
-        spack install mpileaks ^mvapich
-        spack install mpileaks ^mpich
-"""
-__all__ = [ 'depends_on', 'extends', 'provides', 'patch', 'version' ]
-
-import re
-import inspect
-
-from llnl.util.lang import *
-
-import spack
-import spack.spec
-import spack.error
-import spack.url
-from spack.version import Version
-from spack.patch import Patch
-from spack.spec import Spec, parse_anonymous_spec
-
-
-
-def version(ver, checksum=None, **kwargs):
-    """Adds a version and metadata describing how to fetch it.
-       Metadata is just stored as a dict in the package's versions
-       dictionary.  Package must turn it into a valid fetch strategy
-       later.
-    """
-    pkg = caller_locals()
-    versions = pkg.setdefault('versions', {})
-
-    # special case checksum for backward compatibility
-    if checksum:
-        kwargs['md5'] = checksum
-
-    # Store the kwargs for the package to use later when constructing
-    # a fetch strategy.
-    versions[Version(ver)] = kwargs
-
-
-def depends_on(*specs):
-    """Adds a dependencies local variable in the locals of
-       the calling class, based on args. """
-    pkg = get_calling_package_name()
-    clocals = caller_locals()
-    dependencies = clocals.setdefault('dependencies', {})
-
-    for string in specs:
-        for spec in spack.spec.parse(string):
-            if pkg == spec.name:
-                raise CircularReferenceError('depends_on', pkg)
-            dependencies[spec.name] = spec
-
-
-def extends(spec, **kwargs):
-    """Same as depends_on, but dependency is symlinked into parent prefix.
-
-    This is for Python and other language modules where the module
-    needs to be installed into the prefix of the Python installation.
-    Spack handles this by installing modules into their own prefix,
-    but allowing ONE module version to be symlinked into a parent
-    Python install at a time.
-
-    keyword arguments can be passed to extends() so that extension
-    packages can pass parameters to the extendee's extension
-    mechanism.
-
-    """
-    pkg = get_calling_package_name()
-    clocals = caller_locals()
-    dependencies = clocals.setdefault('dependencies', {})
-    extendees = clocals.setdefault('extendees', {})
-    if extendees:
-        raise RelationError("Packages can extend at most one other package.")
-
-    spec = Spec(spec)
-    if pkg == spec.name:
-        raise CircularReferenceError('extends', pkg)
-    dependencies[spec.name] = spec
-    extendees[spec.name] = (spec, kwargs)
-
-
-def provides(*specs, **kwargs):
-    """Allows packages to provide a virtual dependency.  If a package provides
-       'mpi', other packages can declare that they depend on "mpi", and spack
-       can use the providing package to satisfy the dependency.
-    """
-    pkg = get_calling_package_name()
-    spec_string = kwargs.get('when', pkg)
-    provider_spec = parse_anonymous_spec(spec_string, pkg)
-
-    provided = caller_locals().setdefault("provided", {})
-    for string in specs:
-        for provided_spec in spack.spec.parse(string):
-            if pkg == provided_spec.name:
-                raise CircularReferenceError('depends_on', pkg)
-            provided[provided_spec] = provider_spec
-
-
-def patch(url_or_filename, **kwargs):
-    """Packages can declare patches to apply to source.  You can
-       optionally provide a when spec to indicate that a particular
-       patch should only be applied when the package's spec meets
-       certain conditions (e.g. a particular version).
-    """
-    pkg = get_calling_package_name()
-    level = kwargs.get('level', 1)
-    when_spec = parse_anonymous_spec(kwargs.get('when', pkg), pkg)
-
-    patches = caller_locals().setdefault('patches', {})
-    if when_spec not in patches:
-        patches[when_spec] = [Patch(pkg, url_or_filename, level)]
-    else:
-        # if this spec is identical to some other, then append this
-        # patch to the existing list.
-        patches[when_spec].append(Patch(pkg, url_or_filename, level))
-
-
-def conflicts(*specs):
-    """Packages can declare conflicts with other packages.
-       This can be as specific as you like: use regular spec syntax.
-
-       NOT YET IMPLEMENTED.
-    """
-    # TODO: implement conflicts
-    pass
-
-
-class RelationError(spack.error.SpackError):
-    """This is raised when something is wrong with a package relation."""
-    def __init__(self, relation, message):
-        super(RelationError, self).__init__(message)
-        self.relation = relation
-
-
-class ScopeError(RelationError):
-    """This is raised when a relation is called from outside a spack package."""
-    def __init__(self, relation):
-        super(ScopeError, self).__init__(
-            relation,
-            "Must invoke '%s' from inside a class definition!" % relation)
-
-
-class CircularReferenceError(RelationError):
-    """This is raised when something depends on itself."""
-    def __init__(self, relation, package):
-        super(CircularReferenceError, self).__init__(
-            relation,
-            "Package '%s' cannot pass itself to %s." % (package, relation))
-        self.package = package
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index dffdccaddb..aa13f0422c 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -1,5 +1,5 @@
 ##############################################################################
-# Copyright (c) 2013, Lawrence Livermore National Security, LLC.
+# Copyright (c) 2013-2015, Lawrence Livermore National Security, LLC.
 # Produced at the Lawrence Livermore National Laboratory.
 #
 # This file is part of Spack.
@@ -93,8 +93,11 @@ expansion when it is the first character in an id typed on the command line.
 import sys
 import itertools
 import hashlib
+import base64
 from StringIO import StringIO
 from operator import attrgetter
+from external import yaml
+from external.yaml.error import MarkedYAMLError
 
 import llnl.util.tty as tty
 from llnl.util.lang import *
@@ -110,6 +113,9 @@ from spack.util.string import *
 from spack.util.prefix import Prefix
 from spack.virtual import ProviderIndex
 
+# Valid pattern for an identifier in Spack
+identifier_re = r'\w[\w-]*'
+
 # Convenient names for color formats so that other things can use them
 compiler_color         = '@g'
 version_color          = '@c'
@@ -117,6 +123,7 @@ architecture_color     = '@m'
 enabled_variant_color  = '@B'
 disabled_variant_color = '@r'
 dependency_color       = '@.'
+hash_color             = '@K'
 
 """This map determines the coloring of specs when using color output.
    We make the fields different colors to enhance readability.
@@ -126,7 +133,8 @@ color_formats = {'%' : compiler_color,
                  '=' : architecture_color,
                  '+' : enabled_variant_color,
                  '~' : disabled_variant_color,
-                 '^' : dependency_color }
+                 '^' : dependency_color,
+                 '#' : hash_color }
 
 """Regex used for splitting by spec field separators."""
 _separators = '[%s]' % ''.join(color_formats.keys())
@@ -214,20 +222,24 @@ class CompilerSpec(object):
         return CompilerSpec(compiler_spec_like)
 
 
-    def satisfies(self, other):
+    def satisfies(self, other, strict=False):
         other = self._autospec(other)
         return (self.name == other.name and
-                self.versions.satisfies(other.versions))
+                self.versions.satisfies(other.versions, strict=strict))
 
 
     def constrain(self, other):
+        """Intersect self's versions with other.
+
+        Return whether the CompilerSpec changed.
+        """
         other = self._autospec(other)
 
         # ensure that other will actually constrain this spec.
         if not other.satisfies(self):
             raise UnsatisfiableCompilerSpecError(other, self)
 
-        self.versions.intersect(other.versions)
+        return self.versions.intersect(other.versions)
 
 
     @property
@@ -255,6 +267,18 @@ class CompilerSpec(object):
         return (self.name, self.versions)
 
 
+    def to_dict(self):
+        d = {'name' : self.name}
+        d.update(self.versions.to_dict())
+        return { 'compiler' : d }
+
+
+    @staticmethod
+    def from_dict(d):
+        d = d['compiler']
+        return CompilerSpec(d['name'], VersionList.from_dict(d))
+
+
     def __str__(self):
         out = self.name
         if self.versions and self.versions != _any_version:
@@ -267,7 +291,7 @@ class CompilerSpec(object):
 
 
 @key_ordering
-class Variant(object):
+class VariantSpec(object):
     """Variants are named, build-time options for a package.  Names depend
        on the particular package being built, and each named variant can
        be enabled or disabled.
@@ -282,7 +306,7 @@ class Variant(object):
 
 
     def copy(self):
-        return Variant(self.name, self.enabled)
+        return VariantSpec(self.name, self.enabled)
 
 
     def __str__(self):
@@ -291,9 +315,52 @@ class Variant(object):
 
 
 class VariantMap(HashableMap):
-    def satisfies(self, other):
-        return all(self[key].enabled == other[key].enabled
-                   for key in other if key in self)
+    def __init__(self, spec):
+        super(VariantMap, self).__init__()
+        self.spec = spec
+
+
+    def satisfies(self, other, strict=False):
+        if strict or self.spec._concrete:
+            return all(k in self and self[k].enabled == other[k].enabled
+                       for k in other)
+        else:
+            return all(self[k].enabled == other[k].enabled
+                       for k in other if k in self)
+
+
+    def constrain(self, other):
+        """Add all variants in other that aren't in self to self.
+
+        Raises an error if any common variants don't match.
+        Return whether the spec changed.
+        """
+        if other.spec._concrete:
+            for k in self:
+                if k not in other:
+                    raise UnsatisfiableVariantSpecError(self[k], '<absent>')
+
+        changed = False
+        for k in other:
+            if k in self:
+                if self[k].enabled != other[k].enabled:
+                    raise UnsatisfiableVariantSpecError(self[k], other[k])
+            else:
+                self[k] = other[k].copy()
+                changed =True
+        return changed
+
+    @property
+    def concrete(self):
+        return self.spec._concrete or all(
+            v in self for v in self.spec.package.variants)
+
+
+    def copy(self):
+        clone = VariantMap(None)
+        for name, variant in self.items():
+            clone[name] = variant.copy()
+        return clone
 
 
     def __str__(self):
@@ -340,10 +407,11 @@ class Spec(object):
         self.name = other.name
         self.dependents = other.dependents
         self.versions = other.versions
-        self.variants = other.variants
         self.architecture = other.architecture
         self.compiler = other.compiler
         self.dependencies = other.dependencies
+        self.variants = other.variants
+        self.variants.spec = self
 
         # Specs are by default not assumed to be normal, but in some
         # cases we've read them from a file want to assume normal.
@@ -372,7 +440,7 @@ class Spec(object):
         """Called by the parser to add a variant."""
         if name in self.variants: raise DuplicateVariantError(
                 "Cannot specify variant '%s' twice" % name)
-        self.variants[name] = Variant(name, enabled)
+        self.variants[name] = VariantSpec(name, enabled)
 
 
     def _set_compiler(self, compiler):
@@ -436,14 +504,15 @@ class Spec(object):
     @property
     def concrete(self):
         """A spec is concrete if it can describe only ONE build of a package.
-           If any of the name, version, architecture, compiler, or depdenencies
-           are ambiguous,then it is not concrete.
+           If any of the name, version, architecture, compiler,
+           variants, or depdenencies are ambiguous,then it is not concrete.
         """
         if self._concrete:
             return True
 
         self._concrete = bool(not self.virtual
                               and self.versions.concrete
+                              and self.variants.concrete
                               and self.architecture
                               and self.compiler and self.compiler.concrete
                               and self.dependencies.concrete)
@@ -564,18 +633,91 @@ class Spec(object):
         return Prefix(spack.install_layout.path_for_spec(self))
 
 
-    def dep_hash(self, length=None):
-        """Return a hash representing all dependencies of this spec
-           (direct and indirect).
+    def dag_hash(self, length=None):
+        """Return a hash of the entire spec DAG, including connectivity."""
+        yaml_text = yaml.dump(
+            self.to_node_dict(), default_flow_style=True, width=sys.maxint)
+        sha = hashlib.sha1(yaml_text)
+        return base64.b32encode(sha.digest()).lower()[:length]
+
+
+    def to_node_dict(self):
+        d = {
+            'variants' : dict(
+                (name,v.enabled) for name, v in self.variants.items()),
+            'arch' : self.architecture,
+            'dependencies' : dict((d, self.dependencies[d].dag_hash())
+                                  for d in sorted(self.dependencies))
+        }
+        if self.compiler:
+            d.update(self.compiler.to_dict())
+        else:
+            d['compiler'] = None
+        d.update(self.versions.to_dict())
+        return { self.name : d }
+
+
+    def to_yaml(self, stream=None):
+        node_list = []
+        for s in self.traverse(order='pre'):
+            node = s.to_node_dict()
+            node[s.name]['hash'] = s.dag_hash()
+            node_list.append(node)
+        return yaml.dump({ 'spec' : node_list },
+                         stream=stream, default_flow_style=False)
+
+
+    @staticmethod
+    def from_node_dict(node):
+        name = next(iter(node))
+        node = node[name]
+
+        spec = Spec(name)
+        spec.versions = VersionList.from_dict(node)
+        spec.architecture = node['arch']
+
+        if node['compiler'] is None:
+            spec.compiler = None
+        else:
+            spec.compiler = CompilerSpec.from_dict(node)
+
+        for name, enabled in node['variants'].items():
+            spec.variants[name] = VariantSpec(name, enabled)
+
+        return spec
+
+
+    @staticmethod
+    def from_yaml(stream):
+        """Construct a spec from YAML.
+
+        Parameters:
+        stream -- string or file object to read from.
+
+        TODO: currently discards hashes. Include hashes when they
+        represent more than the DAG does.
 
-           If you want this hash to be consistent, you should
-           concretize the spec first so that it is not ambiguous.
         """
-        sha = hashlib.sha1()
-        sha.update(self.dep_string())
-        full_hash = sha.hexdigest()
+        deps = {}
+        spec = None
 
-        return full_hash[:length]
+        try:
+            yfile = yaml.load(stream)
+        except MarkedYAMLError, e:
+            raise SpackYAMLError("error parsing YMAL spec:", str(e))
+
+        for node in yfile['spec']:
+            name = next(iter(node))
+            dep = Spec.from_node_dict(node)
+            if not spec:
+                spec = dep
+            deps[dep.name] = dep
+
+        for node in yfile['spec']:
+            name = next(iter(node))
+            for dep_name in node[name]['dependencies']:
+                deps[name].dependencies[dep_name] = deps[dep_name]
+        return spec
 
 
     def _concretize_helper(self, presets=None, visited=None):
@@ -604,6 +746,7 @@ class Spec(object):
                 spack.concretizer.concretize_architecture(self)
                 spack.concretizer.concretize_compiler(self)
                 spack.concretizer.concretize_version(self)
+                spack.concretizer.concretize_variants(self)
             presets[self.name] = self
 
         visited.add(self.name)
@@ -736,80 +879,156 @@ class Spec(object):
             self._add_dependency(dep)
 
 
+    def _evaluate_dependency_conditions(self, name):
+        """Evaluate all the conditions on a dependency with this name.
+
+        If the package depends on <name> in this configuration, return
+        the dependency.  If no conditions are True (and we don't
+        depend on it), return None.
+        """
+        pkg = spack.db.get(self.name)
+        conditions = pkg.dependencies[name]
+
+        # evaluate when specs to figure out constraints on the dependency.
+        dep = None
+        for when_spec, dep_spec in conditions.items():
+            sat = self.satisfies(when_spec, strict=True)
+            if sat:
+                if dep is None:
+                    dep = Spec(name)
+                try:
+                    dep.constrain(dep_spec)
+                except UnsatisfiableSpecError, e:
+                    e.message = ("Conflicting conditional dependencies on package "
+                                 "%s for spec %s" % (self.name, self))
+                    raise e
+        return dep
+
+
+    def _find_provider(self, vdep, provider_index):
+        """Find provider for a virtual spec in the provider index.
+        Raise an exception if there is a conflicting virtual
+        dependency already in this spec.
+        """
+        assert(vdep.virtual)
+        providers = provider_index.providers_for(vdep)
+
+        # If there is a provider for the vpkg, then use that instead of
+        # the virtual package.
+        if providers:
+            # Can't have multiple providers for the same thing in one spec.
+            if len(providers) > 1:
+                raise MultipleProviderError(vdep, providers)
+            return providers[0]
+        else:
+            # The user might have required something insufficient for
+            # pkg_dep -- so we'll get a conflict.  e.g., user asked for
+            # mpi@:1.1 but some package required mpi@2.1:.
+            required = provider_index.providers_for(vdep.name)
+            if len(required) > 1:
+                raise MultipleProviderError(vdep, required)
+            elif required:
+                raise UnsatisfiableProviderSpecError(required[0], vdep)
+
+
+    def _merge_dependency(self, dep, visited, spec_deps, provider_index):
+        """Merge the dependency into this spec.
+
+        This is the core of the normalize() method.  There are a few basic steps:
+
+          * If dep is virtual, evaluate whether it corresponds to an
+            existing concrete dependency, and merge if so.
+
+          * If it's real and it provides some virtual dep, see if it provides
+            what some virtual dependency wants and merge if so.
+
+          * Finally, if none of the above, merge dependency and its
+            constraints into this spec.
+
+        This method returns True if the spec was changed, False otherwise.
+        """
+        changed = False
+
+        # If it's a virtual dependency, try to find a provider and
+        # merge that.
+        if dep.virtual:
+            visited.add(dep.name)
+            provider = self._find_provider(dep, provider_index)
+            if provider:
+                dep = provider
+
+        else:
+            # if it's a real dependency, check whether it provides
+            # something already required in the spec.
+            index = ProviderIndex([dep], restrict=True)
+            for vspec in (v for v in spec_deps.values() if v.virtual):
+                if index.providers_for(vspec):
+                    vspec._replace_with(dep)
+                    del spec_deps[vspec.name]
+                    changed = True
+                else:
+                    required = index.providers_for(vspec.name)
+                    if required:
+                        raise UnsatisfiableProviderSpecError(required[0], dep)
+            provider_index.update(dep)
+
+        # If the spec isn't already in the set of dependencies, clone
+        # it from the package description.
+        if dep.name not in spec_deps:
+            spec_deps[dep.name] = dep.copy()
+
+        # Constrain package information with spec info
+        try:
+            changed |= spec_deps[dep.name].constrain(dep)
+
+        except UnsatisfiableSpecError, e:
+            e.message =  "Invalid spec: '%s'. "
+            e.message += "Package %s requires %s %s, but spec asked for %s"
+            e.message %= (spec_deps[dep.name], dep.name, e.constraint_type,
+                          e.required, e.provided)
+            raise e
+
+        # Add merged spec to my deps and recurse
+        dependency = spec_deps[dep.name]
+        if dep.name not in self.dependencies:
+            self._add_dependency(dependency)
+            changed = True
+
+        changed |= dependency._normalize_helper(visited, spec_deps, provider_index)
+        return changed
+
+
     def _normalize_helper(self, visited, spec_deps, provider_index):
         """Recursive helper function for _normalize."""
         if self.name in visited:
-            return
+            return False
         visited.add(self.name)
 
         # if we descend into a virtual spec, there's nothing more
         # to normalize.  Concretize will finish resolving it later.
         if self.virtual:
-            return
+            return False
+
+        # Combine constraints from package deps with constraints from
+        # the spec, until nothing changes.
+        any_change = False
+        changed = True
 
-        # Combine constraints from package dependencies with
-        # constraints on the spec's dependencies.
         pkg = spack.db.get(self.name)
-        for name, pkg_dep in self.package.dependencies.items():
-            # If it's a virtual dependency, try to find a provider
-            if pkg_dep.virtual:
-                providers = provider_index.providers_for(pkg_dep)
+        while changed:
+            changed = False
+            for dep_name in pkg.dependencies:
+                # Do we depend on dep_name?  If so pkg_dep is not None.
+                pkg_dep = self._evaluate_dependency_conditions(dep_name)
 
-                # If there is a provider for the vpkg, then use that instead of
-                # the virtual package.
-                if providers:
-                    # Can't have multiple providers for the same thing in one spec.
-                    if len(providers) > 1:
-                        raise MultipleProviderError(pkg_dep, providers)
+                # If pkg_dep is a dependency, merge it.
+                if pkg_dep:
+                    changed |= self._merge_dependency(
+                        pkg_dep, visited, spec_deps, provider_index)
 
-                    pkg_dep = providers[0]
-                    name    = pkg_dep.name
+            any_change |= changed
 
-                else:
-                    # The user might have required something insufficient for
-                    # pkg_dep -- so we'll get a conflict.  e.g., user asked for
-                    # mpi@:1.1 but some package required mpi@2.1:.
-                    required = provider_index.providers_for(name)
-                    if len(required) > 1:
-                        raise MultipleProviderError(pkg_dep, required)
-                    elif required:
-                        raise UnsatisfiableProviderSpecError(
-                            required[0], pkg_dep)
-            else:
-                # if it's a real dependency, check whether it provides something
-                # already required in the spec.
-                index = ProviderIndex([pkg_dep], restrict=True)
-                for vspec in (v for v in spec_deps.values() if v.virtual):
-                    if index.providers_for(vspec):
-                        vspec._replace_with(pkg_dep)
-                        del spec_deps[vspec.name]
-                    else:
-                        required = index.providers_for(vspec.name)
-                        if required:
-                            raise UnsatisfiableProviderSpecError(
-                                required[0], pkg_dep)
-                provider_index.update(pkg_dep)
-
-            if name not in spec_deps:
-                # If the spec doesn't reference a dependency that this package
-                # needs, then clone it from the package description.
-                spec_deps[name] = pkg_dep.copy()
-
-            try:
-                # Constrain package information with spec info
-                spec_deps[name].constrain(pkg_dep)
-
-            except UnsatisfiableSpecError, e:
-                e.message =  "Invalid spec: '%s'. "
-                e.message += "Package %s requires %s %s, but spec asked for %s"
-                e.message %= (spec_deps[name], name, e.constraint_type,
-                              e.required, e.provided)
-                raise e
-
-            # Add merged spec to my deps and recurse
-            dependency = spec_deps[name]
-            self._add_dependency(dependency)
-            dependency._normalize_helper(visited, spec_deps, provider_index)
+        return any_change
 
 
     def normalize(self, **kwargs):
@@ -836,19 +1055,14 @@ class Spec(object):
         # Ensure first that all packages & compilers in the DAG exist.
         self.validate_names()
 
-        # Ensure that the package & dep descriptions are consistent & sane
-        if not self.virtual:
-            self.package.validate_dependencies()
-
         # Get all the dependencies into one DependencyMap
         spec_deps = self.flat_dependencies(copy=False)
 
-        # Figure out which of the user-provided deps provide virtual deps.
-        # Remove virtual deps that are already provided by something in the spec
-        spec_packages = [d.package for d in spec_deps.values() if not d.virtual]
-
+        # Initialize index of virtual dependency providers
         index = ProviderIndex(spec_deps.values(), restrict=True)
 
+        # traverse the package DAG and fill out dependencies according
+        # to package files & their 'when' specs
         visited = set()
         self._normalize_helper(visited, spec_deps, index)
 
@@ -856,12 +1070,6 @@ class Spec(object):
         # actually deps of this package.  Raise an error.
         extra = set(spec_deps.keys()).difference(visited)
 
-        # Also subtract out all the packags that provide a needed vpkg
-        vdeps = [v for v in self.package.virtual_dependencies()]
-
-        vpkg_providers = index.providers_for(*vdeps)
-        extra.difference_update(p.name for p in vpkg_providers)
-
         # Anything left over is not a valid part of the spec.
         if extra:
             raise InvalidDependencyException(
@@ -893,10 +1101,18 @@ class Spec(object):
                 if not compilers.supported(spec.compiler):
                     raise UnsupportedCompilerError(spec.compiler.name)
 
+            # Ensure that variants all exist.
+            for vname, variant in spec.variants.items():
+                if vname not in spec.package.variants:
+                    raise UnknownVariantError(spec.name, vname)
 
-    def constrain(self, other, **kwargs):
+
+    def constrain(self, other, deps=True):
+        """Merge the constraints of other with self.
+
+        Returns True if the spec changed as a result, False if not.
+        """
         other = self._autospec(other)
-        constrain_deps = kwargs.get('deps', True)
 
         if not self.name == other.name:
             raise UnsatisfiableSpecNameError(self.name, other.name)
@@ -915,23 +1131,32 @@ class Spec(object):
                 raise UnsatisfiableArchitectureSpecError(self.architecture,
                                                          other.architecture)
 
+        changed = False
         if self.compiler is not None and other.compiler is not None:
-            self.compiler.constrain(other.compiler)
+            changed |= self.compiler.constrain(other.compiler)
         elif self.compiler is None:
+            changed |= (self.compiler != other.compiler)
             self.compiler = other.compiler
 
-        self.versions.intersect(other.versions)
-        self.variants.update(other.variants)
+        changed |= self.versions.intersect(other.versions)
+        changed |= self.variants.constrain(other.variants)
+
+        old = self.architecture
         self.architecture = self.architecture or other.architecture
+        changed |= (self.architecture != old)
 
-        if constrain_deps:
-            self._constrain_dependencies(other)
+        if deps:
+            changed |= self._constrain_dependencies(other)
+
+        return changed
 
 
     def _constrain_dependencies(self, other):
         """Apply constraints of other spec's dependencies to this spec."""
+        other = self._autospec(other)
+
         if not self.dependencies or not other.dependencies:
-            return
+            return False
 
         # TODO: might want more detail than this, e.g. specific deps
         # in violation. if this becomes a priority get rid of this
@@ -940,12 +1165,17 @@ class Spec(object):
             raise UnsatisfiableDependencySpecError(other, self)
 
         # Handle common first-order constraints directly
+        changed = False
         for name in self.common_dependencies(other):
-            self[name].constrain(other[name], deps=False)
+            changed |= self[name].constrain(other[name], deps=False)
+
 
         # Update with additional constraints from other spec
         for name in other.dep_difference(self):
             self._add_dependency(other[name].copy())
+            changed = True
+
+        return changed
 
 
     def common_dependencies(self, other):
@@ -979,44 +1209,74 @@ class Spec(object):
             return parse_anonymous_spec(spec_like, self.name)
 
 
-    def satisfies(self, other, **kwargs):
+    def satisfies(self, other, deps=True, strict=False):
+        """Determine if this spec satisfies all constraints of another.
+
+        There are two senses for satisfies:
+
+          * `loose` (default): the absence of a constraint in self
+            implies that it *could* be satisfied by other, so we only
+            check that there are no conflicts with other for
+            constraints that this spec actually has.
+
+          * `strict`: strict means that we *must* meet all the
+            constraints specified on other.
+        """
         other = self._autospec(other)
-        satisfy_deps = kwargs.get('deps', True)
 
         # First thing we care about is whether the name matches
         if self.name != other.name:
             return False
 
-        # All these attrs have satisfies criteria of their own,
-        # but can be None to indicate no constraints.
-        for s, o in ((self.versions, other.versions),
-                     (self.variants, other.variants),
-                     (self.compiler, other.compiler)):
-            if s and o and not s.satisfies(o):
+        if self.versions and other.versions:
+            if not self.versions.satisfies(other.versions, strict=strict):
                 return False
+        elif strict and (self.versions or other.versions):
+            return False
+
+        # None indicates no constraints when not strict.
+        if self.compiler and other.compiler:
+            if not self.compiler.satisfies(other.compiler, strict=strict):
+                return False
+        elif strict and (other.compiler and not self.compiler):
+            return False
+
+        if not self.variants.satisfies(other.variants, strict=strict):
+            return False
 
         # Architecture satisfaction is currently just string equality.
-        # Can be None for unconstrained, though.
-        if (self.architecture and other.architecture and
-            self.architecture != other.architecture):
+        # If not strict, None means unconstrained.
+        if self.architecture and other.architecture:
+            if self.architecture != other.architecture:
+                return False
+        elif strict and (other.architecture and not self.architecture):
             return False
 
         # If we need to descend into dependencies, do it, otherwise we're done.
-        if satisfy_deps:
-            return self.satisfies_dependencies(other)
+        if deps:
+            return self.satisfies_dependencies(other, strict=strict)
         else:
             return True
 
 
-    def satisfies_dependencies(self, other):
+    def satisfies_dependencies(self, other, strict=False):
         """This checks constraints on common dependencies against each other."""
-        # if either spec doesn't restrict dependencies then both are compatible.
-        if not self.dependencies or not other.dependencies:
+        other = self._autospec(other)
+
+        if strict:
+            if other.dependencies and not self.dependencies:
+                return False
+
+            if not all(dep in self.dependencies for dep in other.dependencies):
+                return False
+
+        elif not self.dependencies or not other.dependencies:
+            # if either spec doesn't restrict dependencies then both are compatible.
             return True
 
         # Handle first-order constraints directly
         for name in self.common_dependencies(other):
-            if not self[name].satisfies(other[name]):
+            if not self[name].satisfies(other[name], deps=False):
                 return False
 
         # For virtual dependencies, we need to dig a little deeper.
@@ -1061,11 +1321,12 @@ class Spec(object):
         # Local node attributes get copied first.
         self.name = other.name
         self.versions = other.versions.copy()
-        self.variants = other.variants.copy()
         self.architecture = other.architecture
         self.compiler = other.compiler.copy() if other.compiler else None
         self.dependents = DependencyMap()
         self.dependencies = DependencyMap()
+        self.variants = other.variants.copy()
+        self.variants.spec = self
 
         # If we copy dependencies, preserve DAG structure in the new spec
         if kwargs.get('deps', True):
@@ -1117,7 +1378,7 @@ class Spec(object):
         """
         spec = self._autospec(spec)
         for s in self.traverse():
-            if s.satisfies(spec):
+            if s.satisfies(spec, strict=True):
                 return True
         return False
 
@@ -1208,7 +1469,7 @@ class Spec(object):
                $%@  Compiler & compiler version
                $+   Options
                $=   Architecture
-               $#   Dependencies' 8-char sha1 prefix
+               $#   7-char prefix of DAG hash
                $$   $
 
            Optionally you can provide a width, e.g. $20_ for a 20-wide name.
@@ -1264,8 +1525,7 @@ class Spec(object):
                     if self.architecture:
                         write(fmt % (c + str(self.architecture)), c)
                 elif c == '#':
-                    if self.dependencies:
-                        out.write(fmt % ('-' + self.dep_hash(8)))
+                    out.write('-' + fmt % (self.dag_hash(7)))
                 elif c == '$':
                     if fmt != '':
                         raise ValueError("Can't use format width with $$.")
@@ -1274,7 +1534,8 @@ class Spec(object):
 
             elif compiler:
                 if c == '@':
-                    if self.compiler and self.compiler.versions:
+                    if (self.compiler and self.compiler.versions and
+                        self.compiler.versions != _any_version):
                         write(c + str(self.compiler.versions), '%')
                 elif c == '$':
                     escape = True
@@ -1311,12 +1572,15 @@ class Spec(object):
         cover  = kwargs.pop('cover', 'nodes')
         indent = kwargs.pop('indent', 0)
         fmt    = kwargs.pop('format', '$_$@$%@$+$=')
+        prefix = kwargs.pop('prefix', None)
         check_kwargs(kwargs, self.tree)
 
         out = ""
         cur_id = 0
         ids = {}
         for d, node in self.traverse(order='pre', cover=cover, depth=True):
+            if prefix is not None:
+                out += prefix(node)
             out += " " * indent
             if depth:
                 out += "%-4d" % d
@@ -1354,6 +1618,8 @@ class SpecLexer(spack.parse.Lexer):
             (r'\~',        lambda scanner, val: self.token(OFF,   val)),
             (r'\%',        lambda scanner, val: self.token(PCT,   val)),
             (r'\=',        lambda scanner, val: self.token(EQ,    val)),
+            # This is more liberal than identifier_re (see above).
+            # Checked by check_identifier() for better error messages.
             (r'\w[\w.-]*', lambda scanner, val: self.token(ID,    val)),
             (r'\s+',       lambda scanner, val: None)])
 
@@ -1399,7 +1665,7 @@ class SpecParser(spack.parse.Parser):
         spec = Spec.__new__(Spec)
         spec.name = self.token.value
         spec.versions = VersionList()
-        spec.variants = VariantMap()
+        spec.variants = VariantMap(spec)
         spec.architecture = None
         spec.compiler = None
         spec.dependents   = DependencyMap()
@@ -1580,6 +1846,13 @@ class UnsupportedCompilerError(SpecError):
             "The '%s' compiler is not yet supported." % compiler_name)
 
 
+class UnknownVariantError(SpecError):
+    """Raised when the same variant occurs in a spec twice."""
+    def __init__(self, pkg, variant):
+        super(UnknownVariantError, self).__init__(
+            "Package %s has no variant %s!" % (pkg, variant))
+
+
 class DuplicateArchitectureError(SpecError):
     """Raised when the same architecture occurs in a spec twice."""
     def __init__(self, message):
@@ -1683,3 +1956,7 @@ class UnsatisfiableDependencySpecError(UnsatisfiableSpecError):
     def __init__(self, provided, required):
         super(UnsatisfiableDependencySpecError, self).__init__(
             provided, required, "dependency")
+
+class SpackYAMLError(spack.error.SpackError):
+    def __init__(self, msg, yaml_error):
+        super(SpackError, self).__init__(msg, str(yaml_error))
diff --git a/lib/spack/spack/stage.py b/lib/spack/spack/stage.py
index d451743508..008c5f0429 100644
--- a/lib/spack/spack/stage.py
+++ b/lib/spack/spack/stage.py
@@ -344,13 +344,9 @@ class DIYStage(object):
 
 def _get_mirrors():
     """Get mirrors from spack configuration."""
-    config = spack.config.get_config()
+    config = spack.config.get_mirror_config()
+    return [val for name, val in config.iteritems()]
 
-    mirrors = []
-    sec_names = config.get_section_names('mirror')
-    for name in sec_names:
-        mirrors.append(config.get_value('mirror', name, 'url'))
-    return mirrors
 
 
 def ensure_access(file=spack.stage_path):
diff --git a/lib/spack/spack/test/__init__.py b/lib/spack/spack/test/__init__.py
index c53e6774fc..7ff512c370 100644
--- a/lib/spack/spack/test/__init__.py
+++ b/lib/spack/spack/test/__init__.py
@@ -52,7 +52,9 @@ test_names = ['versions',
               'mirror',
               'url_extrapolate',
               'cc',
-              'link_tree']
+              'link_tree',
+              'spec_yaml',
+              'optional_deps']
 
 
 def list_tests():
diff --git a/lib/spack/spack/test/concretize.py b/lib/spack/spack/test/concretize.py
index a7f4812c8c..cc839a2340 100644
--- a/lib/spack/spack/test/concretize.py
+++ b/lib/spack/spack/test/concretize.py
@@ -35,7 +35,13 @@ class ConcretizeTest(MockPackagesTest):
             self.assertEqual(abstract.versions, concrete.versions)
 
         if abstract.variants:
-            self.assertEqual(abstract.versions, concrete.versions)
+            for name in abstract.variants:
+                avariant = abstract.variants[name]
+                cvariant = concrete.variants[name]
+                self.assertEqual(avariant.enabled, cvariant.enabled)
+
+        for name in abstract.package.variants:
+            self.assertTrue(name in concrete.variants)
 
         if abstract.compiler and abstract.compiler.concrete:
             self.assertEqual(abstract.compiler, concrete.compiler)
@@ -66,6 +72,12 @@ class ConcretizeTest(MockPackagesTest):
         self.check_concretize('libelf')
 
 
+    def test_concretize_variant(self):
+        self.check_concretize('mpich+debug')
+        self.check_concretize('mpich~debug')
+        self.check_concretize('mpich')
+
+
     def test_concretize_with_virtual(self):
         self.check_concretize('mpileaks ^mpi')
         self.check_concretize('mpileaks ^mpi@:1.1')
diff --git a/lib/spack/spack/test/config.py b/lib/spack/spack/test/config.py
index c676e9a35b..790b22f3b0 100644
--- a/lib/spack/spack/test/config.py
+++ b/lib/spack/spack/test/config.py
@@ -26,44 +26,49 @@ import unittest
 import shutil
 import os
 from tempfile import mkdtemp
+import spack
+from spack.packages import PackageDB
+from spack.test.mock_packages_test import *
 
-from spack.config import *
+class ConfigTest(MockPackagesTest):
 
+    def setUp(self):
+        self.initmock()
+        self.tmp_dir = mkdtemp('.tmp', 'spack-config-test-')
+        spack.config.config_scopes = [('test_low_priority', os.path.join(self.tmp_dir, 'low')),
+                                      ('test_high_priority', os.path.join(self.tmp_dir, 'high'))]
 
-class ConfigTest(unittest.TestCase):
+    def tearDown(self):
+        self.cleanmock()
+        shutil.rmtree(self.tmp_dir, True)
 
-    @classmethod
-    def setUp(cls):
-        cls.tmp_dir = mkdtemp('.tmp', 'spack-config-test-')
+    def check_config(self, comps):
+        config = spack.config.get_compilers_config()
+        compiler_list = ['cc', 'cxx', 'f77', 'f90']
+        for key in comps:
+            for c in compiler_list:
+                if comps[key][c] == '/bad':
+                    continue
+                self.assertEqual(comps[key][c], config[key][c])
 
 
-    @classmethod
-    def tearDown(cls):
-        shutil.rmtree(cls.tmp_dir, True)
-
-
-    def get_path(self):
-        return os.path.join(ConfigTest.tmp_dir, "spackconfig")
+    def test_write_key(self):
+        a_comps =  {"gcc@4.7.3" : { "cc" : "/gcc473", "cxx" : "/g++473", "f77" : None, "f90" : None },
+         "gcc@4.5.0" : { "cc" : "/gcc450", "cxx" : "/g++450", "f77" : "/gfortran", "f90" : "/gfortran" },
+         "clang@3.3"  : { "cc" : "/bad", "cxx" : "/bad", "f77" : "/bad", "f90" : "/bad" }}
 
+        b_comps = {"icc@10.0" : { "cc" : "/icc100", "cxx" : "/icc100", "f77" : None, "f90" : None },
+         "icc@11.1" : { "cc" : "/icc111", "cxx" : "/icp111", "f77" : "/ifort", "f90" : "/ifort" },
+         "clang@3.3" : { "cc" : "/clang", "cxx" : "/clang++", "f77" :  None, "f90" : None}}
 
-    def test_write_key(self):
-        config = SpackConfigParser(self.get_path())
-        config.set_value('compiler.cc',  'a')
-        config.set_value('compiler.cxx', 'b')
-        config.set_value('compiler', 'gcc@4.7.3', 'cc',  'c')
-        config.set_value('compiler', 'gcc@4.7.3', 'cxx', 'd')
-        config.write()
+        spack.config.add_to_compiler_config(a_comps, 'test_low_priority')
+        spack.config.add_to_compiler_config(b_comps, 'test_high_priority')
 
-        config = SpackConfigParser(self.get_path())
+        self.check_config(a_comps)
+        self.check_config(b_comps)
 
-        self.assertEqual(config.get_value('compiler.cc'),  'a')
-        self.assertEqual(config.get_value('compiler.cxx'), 'b')
-        self.assertEqual(config.get_value('compiler', 'gcc@4.7.3', 'cc'), 'c')
-        self.assertEqual(config.get_value('compiler', 'gcc@4.7.3', 'cxx'), 'd')
+        spack.config.clear_config_caches()
 
-        self.assertEqual(config.get_value('compiler', None, 'cc'),  'a')
-        self.assertEqual(config.get_value('compiler', None, 'cxx'), 'b')
-        self.assertEqual(config.get_value('compiler.gcc@4.7.3.cc'), 'c')
-        self.assertEqual(config.get_value('compiler.gcc@4.7.3.cxx'), 'd')
+        self.check_config(a_comps)
+        self.check_config(b_comps)
 
-        self.assertRaises(NoOptionError, config.get_value, 'compiler', None, 'fc')
diff --git a/lib/spack/spack/test/directory_layout.py b/lib/spack/spack/test/directory_layout.py
index 3e52954cfe..7ca84090f2 100644
--- a/lib/spack/spack/test/directory_layout.py
+++ b/lib/spack/spack/test/directory_layout.py
@@ -36,7 +36,11 @@ from llnl.util.filesystem import *
 import spack
 from spack.spec import Spec
 from spack.packages import PackageDB
-from spack.directory_layout import SpecHashDirectoryLayout
+from spack.directory_layout import YamlDirectoryLayout
+
+# number of packages to test (to reduce test time)
+max_packages = 10
+
 
 class DirectoryLayoutTest(unittest.TestCase):
     """Tests that a directory layout works correctly and produces a
@@ -44,11 +48,11 @@ class DirectoryLayoutTest(unittest.TestCase):
 
     def setUp(self):
         self.tmpdir = tempfile.mkdtemp()
-        self.layout = SpecHashDirectoryLayout(self.tmpdir)
+        self.layout = YamlDirectoryLayout(self.tmpdir)
 
 
     def tearDown(self):
-        shutil.rmtree(self.tmpdir, ignore_errors=True)
+        #shutil.rmtree(self.tmpdir, ignore_errors=True)
         self.layout = None
 
 
@@ -59,7 +63,9 @@ class DirectoryLayoutTest(unittest.TestCase):
            finally that the directory can be removed by the directory
            layout.
         """
-        for pkg in spack.db.all_packages():
+        packages = list(spack.db.all_packages())[:max_packages]
+
+        for pkg in packages:
             spec = pkg.spec
 
             # If a spec fails to concretize, just skip it.  If it is a
@@ -69,7 +75,7 @@ class DirectoryLayoutTest(unittest.TestCase):
             except:
                 continue
 
-            self.layout.make_path_for_spec(spec)
+            self.layout.create_install_directory(spec)
 
             install_dir = self.layout.path_for_spec(spec)
             spec_path = self.layout.spec_file_path(spec)
@@ -90,7 +96,7 @@ class DirectoryLayoutTest(unittest.TestCase):
 
             # Ensure that specs that come out "normal" are really normal.
             with closing(open(spec_path)) as spec_file:
-                read_separately = Spec(spec_file.read())
+                read_separately = Spec.from_yaml(spec_file.read())
 
                 read_separately.normalize()
                 self.assertEqual(read_separately, spec_from_file)
@@ -98,11 +104,11 @@ class DirectoryLayoutTest(unittest.TestCase):
                 read_separately.concretize()
                 self.assertEqual(read_separately, spec_from_file)
 
-            # Make sure the dep hash of the read-in spec is the same
-            self.assertEqual(spec.dep_hash(), spec_from_file.dep_hash())
+            # Make sure the hash of the read-in spec is the same
+            self.assertEqual(spec.dag_hash(), spec_from_file.dag_hash())
 
             # Ensure directories are properly removed
-            self.layout.remove_path_for_spec(spec)
+            self.layout.remove_install_directory(spec)
             self.assertFalse(os.path.isdir(install_dir))
             self.assertFalse(os.path.exists(install_dir))
 
@@ -120,12 +126,14 @@ class DirectoryLayoutTest(unittest.TestCase):
         """
         mock_db = PackageDB(spack.mock_packages_path)
 
-        not_in_mock = set(spack.db.all_package_names()).difference(
+        not_in_mock = set.difference(
+            set(spack.db.all_package_names()),
             set(mock_db.all_package_names()))
+        packages = list(not_in_mock)[:max_packages]
 
         # Create all the packages that are not in mock.
         installed_specs = {}
-        for pkg_name in not_in_mock:
+        for pkg_name in packages:
             spec = spack.db.get(pkg_name).spec
 
             # If a spec fails to concretize, just skip it.  If it is a
@@ -135,7 +143,7 @@ class DirectoryLayoutTest(unittest.TestCase):
             except:
                 continue
 
-            self.layout.make_path_for_spec(spec)
+            self.layout.create_install_directory(spec)
             installed_specs[spec] = self.layout.path_for_spec(spec)
 
         tmp = spack.db
@@ -144,12 +152,29 @@ class DirectoryLayoutTest(unittest.TestCase):
         # Now check that even without the package files, we know
         # enough to read a spec from the spec file.
         for spec, path in installed_specs.items():
-            spec_from_file = self.layout.read_spec(join_path(path, '.spec'))
+            spec_from_file = self.layout.read_spec(
+                join_path(path, '.spack', 'spec.yaml'))
 
             # To satisfy these conditions, directory layouts need to
             # read in concrete specs from their install dirs somehow.
             self.assertEqual(path, self.layout.path_for_spec(spec_from_file))
             self.assertEqual(spec, spec_from_file)
-            self.assertEqual(spec.dep_hash(), spec_from_file.dep_hash())
+            self.assertTrue(spec.eq_dag(spec_from_file))
+            self.assertEqual(spec.dag_hash(), spec_from_file.dag_hash())
 
         spack.db = tmp
+
+
+    def test_find(self):
+        """Test that finding specs within an install layout works."""
+        packages = list(spack.db.all_packages())[:max_packages]
+        installed_specs = {}
+        for pkg in packages:
+            spec = pkg.spec.concretized()
+            installed_specs[spec.name] = spec
+            self.layout.create_install_directory(spec)
+
+        found_specs = dict((s.name, s) for s in self.layout.all_specs())
+        for name, spec in found_specs.items():
+            self.assertTrue(name in found_specs)
+            self.assertTrue(found_specs[name].eq_dag(spec))
diff --git a/lib/spack/spack/test/install.py b/lib/spack/spack/test/install.py
index e052f53e77..d240a393a6 100644
--- a/lib/spack/spack/test/install.py
+++ b/lib/spack/spack/test/install.py
@@ -33,7 +33,7 @@ from llnl.util.filesystem import *
 import spack
 from spack.stage import Stage
 from spack.fetch_strategy import URLFetchStrategy
-from spack.directory_layout import SpecHashDirectoryLayout
+from spack.directory_layout import YamlDirectoryLayout
 from spack.util.executable import which
 from spack.test.mock_packages_test import *
 from spack.test.mock_repo import MockArchive
@@ -55,7 +55,7 @@ class InstallTest(MockPackagesTest):
         # installed pkgs and mock packages.
         self.tmpdir = tempfile.mkdtemp()
         self.orig_layout = spack.install_layout
-        spack.install_layout = SpecHashDirectoryLayout(self.tmpdir)
+        spack.install_layout = YamlDirectoryLayout(self.tmpdir)
 
 
     def tearDown(self):
diff --git a/lib/spack/spack/test/mock_packages_test.py b/lib/spack/spack/test/mock_packages_test.py
index e948376039..00f81114af 100644
--- a/lib/spack/spack/test/mock_packages_test.py
+++ b/lib/spack/spack/test/mock_packages_test.py
@@ -31,29 +31,40 @@ from spack.spec import Spec
 
 
 def set_pkg_dep(pkg, spec):
-    """Alters dependence information for a pacakge.
+    """Alters dependence information for a package.
        Use this to mock up constraints.
     """
     spec = Spec(spec)
-    spack.db.get(pkg).dependencies[spec.name] = spec
+    spack.db.get(pkg).dependencies[spec.name] = { Spec(pkg) : spec }
 
 
 class MockPackagesTest(unittest.TestCase):
-    def setUp(self):
+    def initmock(self):
         # Use the mock packages database for these tests.  This allows
         # us to set up contrived packages that don't interfere with
         # real ones.
         self.real_db = spack.db
         spack.db = PackageDB(spack.mock_packages_path)
 
-        self.real_scopes = spack.config._scopes
-        spack.config._scopes = {
-            'site' : spack.mock_site_config,
-            'user' : spack.mock_user_config }
+        spack.config.clear_config_caches()
+        self.real_scopes = spack.config.config_scopes
+        spack.config.config_scopes = [
+            ('site', spack.mock_site_config),
+            ('user', spack.mock_user_config)]
 
 
-    def tearDown(self):
+    def cleanmock(self):
         """Restore the real packages path after any test."""
         spack.db = self.real_db
-        spack.config._scopes = self.real_scopes
+        spack.config.config_scopes = self.real_scopes
+        spack.config.clear_config_caches()
+
+
+    def setUp(self):
+        self.initmock()
+
+
+    def tearDown(self):
+        self.cleanmock()
+
 
diff --git a/lib/spack/spack/test/optional_deps.py b/lib/spack/spack/test/optional_deps.py
new file mode 100644
index 0000000000..265a983f3f
--- /dev/null
+++ b/lib/spack/spack/test/optional_deps.py
@@ -0,0 +1,94 @@
+##############################################################################
+# Copyright (c) 2013-2015, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://scalability-llnl.github.io/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+import unittest
+
+import spack
+from spack.spec import Spec, CompilerSpec
+from spack.test.mock_packages_test import *
+
+class ConcretizeTest(MockPackagesTest):
+
+    def check_normalize(self, spec_string, expected):
+        spec = Spec(spec_string)
+        spec.normalize()
+        self.assertEqual(spec, expected)
+        self.assertTrue(spec.eq_dag(expected))
+
+
+    def test_normalize_simple_conditionals(self):
+        self.check_normalize('optional-dep-test', Spec('optional-dep-test'))
+        self.check_normalize('optional-dep-test~a', Spec('optional-dep-test~a'))
+
+        self.check_normalize('optional-dep-test+a',
+                             Spec('optional-dep-test+a', Spec('a')))
+
+        self.check_normalize('optional-dep-test@1.1',
+                             Spec('optional-dep-test@1.1', Spec('b')))
+
+        self.check_normalize('optional-dep-test%intel',
+                             Spec('optional-dep-test%intel', Spec('c')))
+
+        self.check_normalize('optional-dep-test%intel@64.1',
+                             Spec('optional-dep-test%intel@64.1', Spec('c'), Spec('d')))
+
+        self.check_normalize('optional-dep-test%intel@64.1.2',
+                             Spec('optional-dep-test%intel@64.1.2', Spec('c'), Spec('d')))
+
+        self.check_normalize('optional-dep-test%clang@35',
+                             Spec('optional-dep-test%clang@35', Spec('e')))
+
+
+    def test_multiple_conditionals(self):
+        self.check_normalize('optional-dep-test+a@1.1',
+                             Spec('optional-dep-test+a@1.1', Spec('a'), Spec('b')))
+
+        self.check_normalize('optional-dep-test+a%intel',
+                             Spec('optional-dep-test+a%intel', Spec('a'), Spec('c')))
+
+        self.check_normalize('optional-dep-test@1.1%intel',
+                             Spec('optional-dep-test@1.1%intel', Spec('b'), Spec('c')))
+
+        self.check_normalize('optional-dep-test@1.1%intel@64.1.2+a',
+                             Spec('optional-dep-test@1.1%intel@64.1.2+a',
+                                  Spec('b'), Spec('a'), Spec('c'), Spec('d')))
+
+        self.check_normalize('optional-dep-test@1.1%clang@36.5+a',
+                             Spec('optional-dep-test@1.1%clang@36.5+a',
+                                  Spec('b'), Spec('a'), Spec('e')))
+
+
+    def test_chained_mpi(self):
+        self.check_normalize('optional-dep-test-2+mpi',
+                             Spec('optional-dep-test-2+mpi',
+                                  Spec('optional-dep-test+mpi',
+                                       Spec('mpi'))))
+
+
+    def test_transitive_chain(self):
+        # Each of these dependencies comes from a conditional
+        # dependency on another.  This requires iterating to evaluate
+        # the whole chain.
+        self.check_normalize('optional-dep-test+f',
+                             Spec('optional-dep-test+f', Spec('f'), Spec('g'), Spec('mpi')))
diff --git a/lib/spack/spack/test/spec_dag.py b/lib/spack/spack/test/spec_dag.py
index fb67aa8a8d..549f829d3e 100644
--- a/lib/spack/spack/test/spec_dag.py
+++ b/lib/spack/spack/test/spec_dag.py
@@ -44,8 +44,11 @@ class SpecDagTest(MockPackagesTest):
         set_pkg_dep('callpath', 'mpich@2.0')
 
         spec = Spec('mpileaks ^mpich ^callpath ^dyninst ^libelf ^libdwarf')
-        self.assertRaises(spack.package.InvalidPackageDependencyError,
-                          spec.package.validate_dependencies)
+
+        # TODO: try to do something to showt that the issue was with
+        # TODO: the user's input or with package inconsistencies.
+        self.assertRaises(spack.spec.UnsatisfiableVersionSpecError,
+                          spec.normalize)
 
 
     def test_preorder_node_traversal(self):
@@ -140,11 +143,6 @@ class SpecDagTest(MockPackagesTest):
 
     def test_conflicting_spec_constraints(self):
         mpileaks = Spec('mpileaks ^mpich ^callpath ^dyninst ^libelf ^libdwarf')
-        try:
-            mpileaks.package.validate_dependencies()
-        except spack.package.InvalidPackageDependencyError, e:
-            self.fail("validate_dependencies raised an exception: %s"
-                      % e.message)
 
         # Normalize then add conflicting constraints to the DAG (this is an
         # extremely unlikely scenario, but we test for it anyway)
@@ -242,12 +240,6 @@ class SpecDagTest(MockPackagesTest):
         self.assertRaises(spack.spec.UnsatisfiableCompilerSpecError, spec.normalize)
 
 
-    def test_unsatisfiable_variant(self):
-        set_pkg_dep('mpileaks', 'mpich+debug')
-        spec = Spec('mpileaks ^mpich~debug ^callpath ^dyninst ^libelf ^libdwarf')
-        self.assertRaises(spack.spec.UnsatisfiableVariantSpecError, spec.normalize)
-
-
     def test_unsatisfiable_architecture(self):
         set_pkg_dep('mpileaks', 'mpich=bgqos_0')
         spec = Spec('mpileaks ^mpich=sles_10_ppc64 ^callpath ^dyninst ^libelf ^libdwarf')
diff --git a/lib/spack/spack/test/spec_semantics.py b/lib/spack/spack/test/spec_semantics.py
index 5fb09e68af..20df2603f5 100644
--- a/lib/spack/spack/test/spec_semantics.py
+++ b/lib/spack/spack/test/spec_semantics.py
@@ -33,8 +33,8 @@ class SpecSematicsTest(MockPackagesTest):
     # ================================================================================
     # Utility functions to set everything up.
     # ================================================================================
-    def check_satisfies(self, spec, anon_spec):
-        left = Spec(spec)
+    def check_satisfies(self, spec, anon_spec, concrete=False):
+        left = Spec(spec, concrete=concrete)
         right = parse_anonymous_spec(anon_spec, left.name)
 
         # Satisfies is one-directional.
@@ -46,8 +46,8 @@ class SpecSematicsTest(MockPackagesTest):
         right.copy().constrain(left)
 
 
-    def check_unsatisfiable(self, spec, anon_spec):
-        left = Spec(spec)
+    def check_unsatisfiable(self, spec, anon_spec, concrete=False):
+        left = Spec(spec, concrete=concrete)
         right = parse_anonymous_spec(anon_spec, left.name)
 
         self.assertFalse(left.satisfies(right))
@@ -64,6 +64,16 @@ class SpecSematicsTest(MockPackagesTest):
         self.assertEqual(exp, spec)
 
 
+    def check_constrain_changed(self, spec, constraint):
+        spec = Spec(spec)
+        self.assertTrue(spec.constrain(constraint))
+
+
+    def check_constrain_not_changed(self, spec, constraint):
+        spec = Spec(spec)
+        self.assertFalse(spec.constrain(constraint))
+
+
     def check_invalid_constraint(self, spec, constraint):
         spec = Spec(spec)
         constraint = Spec(constraint)
@@ -71,7 +81,7 @@ class SpecSematicsTest(MockPackagesTest):
 
 
     # ================================================================================
-    # Satisfiability and constraints
+    # Satisfiability
     # ================================================================================
     def test_satisfies(self):
         self.check_satisfies('libelf@0.8.13', '@0:1')
@@ -96,6 +106,9 @@ class SpecSematicsTest(MockPackagesTest):
         self.check_unsatisfiable('foo@4.0%pgi', '@1:3%pgi')
         self.check_unsatisfiable('foo@4.0%pgi@4.5', '@1:3%pgi@4.4:4.6')
 
+        self.check_satisfies('foo %gcc@4.7.3', '%gcc@4.7')
+        self.check_unsatisfiable('foo %gcc@4.7', '%gcc@4.7.3')
+
 
     def test_satisfies_architecture(self):
         self.check_satisfies('foo=chaos_5_x86_64_ib', '=chaos_5_x86_64_ib')
@@ -147,7 +160,40 @@ class SpecSematicsTest(MockPackagesTest):
         self.check_unsatisfiable('mpileaks^mpi@3:', '^mpich@1.0')
 
 
-    def test_constrain(self):
+    def test_satisfies_matching_variant(self):
+        self.check_satisfies('mpich+foo', 'mpich+foo')
+        self.check_satisfies('mpich~foo', 'mpich~foo')
+
+
+    def test_satisfies_unconstrained_variant(self):
+        # only asked for mpich, no constraints.  Either will do.
+        self.check_satisfies('mpich+foo', 'mpich')
+        self.check_satisfies('mpich~foo', 'mpich')
+
+
+    def test_unsatisfiable_variants(self):
+        # This case is different depending on whether the specs are concrete.
+
+        # 'mpich' is not concrete:
+        self.check_satisfies('mpich', 'mpich+foo', False)
+        self.check_satisfies('mpich', 'mpich~foo', False)
+
+        # 'mpich' is concrete:
+        self.check_unsatisfiable('mpich', 'mpich+foo', True)
+        self.check_unsatisfiable('mpich', 'mpich~foo', True)
+
+
+    def test_unsatisfiable_variant_mismatch(self):
+        # No matchi in specs
+        self.check_unsatisfiable('mpich~foo', 'mpich+foo')
+        self.check_unsatisfiable('mpich+foo', 'mpich~foo')
+
+
+
+    # ================================================================================
+    # Constraints
+    # ================================================================================
+    def test_constrain_variants(self):
         self.check_constrain('libelf@2.1:2.5', 'libelf@0:2.5', 'libelf@2.1:3')
         self.check_constrain('libelf@2.1:2.5%gcc@4.5:4.6',
                              'libelf@0:2.5%gcc@2:4.6', 'libelf@2.1:3%gcc@4.5:4.7')
@@ -158,6 +204,13 @@ class SpecSematicsTest(MockPackagesTest):
         self.check_constrain('libelf+debug~foo', 'libelf+debug', 'libelf~foo')
         self.check_constrain('libelf+debug~foo', 'libelf+debug', 'libelf+debug~foo')
 
+
+    def test_constrain_arch(self):
+        self.check_constrain('libelf=bgqos_0', 'libelf=bgqos_0', 'libelf=bgqos_0')
+        self.check_constrain('libelf=bgqos_0', 'libelf', 'libelf=bgqos_0')
+
+
+    def test_constrain_compiler(self):
         self.check_constrain('libelf=bgqos_0', 'libelf=bgqos_0', 'libelf=bgqos_0')
         self.check_constrain('libelf=bgqos_0', 'libelf', 'libelf=bgqos_0')
 
@@ -172,6 +225,45 @@ class SpecSematicsTest(MockPackagesTest):
         self.check_invalid_constraint('libelf=bgqos_0', 'libelf=x86_54')
 
 
-    def test_compiler_satisfies(self):
-        self.check_satisfies('foo %gcc@4.7.3', '%gcc@4.7')
-        self.check_unsatisfiable('foo %gcc@4.7', '%gcc@4.7.3')
+    def test_constrain_changed(self):
+        self.check_constrain_changed('libelf', '@1.0')
+        self.check_constrain_changed('libelf', '@1.0:5.0')
+        self.check_constrain_changed('libelf', '%gcc')
+        self.check_constrain_changed('libelf%gcc', '%gcc@4.5')
+        self.check_constrain_changed('libelf', '+debug')
+        self.check_constrain_changed('libelf', '~debug')
+        self.check_constrain_changed('libelf', '=bgqos_0')
+
+
+    def test_constrain_not_changed(self):
+        self.check_constrain_not_changed('libelf', 'libelf')
+        self.check_constrain_not_changed('libelf@1.0', '@1.0')
+        self.check_constrain_not_changed('libelf@1.0:5.0', '@1.0:5.0')
+        self.check_constrain_not_changed('libelf%gcc', '%gcc')
+        self.check_constrain_not_changed('libelf%gcc@4.5', '%gcc@4.5')
+        self.check_constrain_not_changed('libelf+debug', '+debug')
+        self.check_constrain_not_changed('libelf~debug', '~debug')
+        self.check_constrain_not_changed('libelf=bgqos_0', '=bgqos_0')
+        self.check_constrain_not_changed('libelf^foo', 'libelf^foo')
+        self.check_constrain_not_changed('libelf^foo^bar', 'libelf^foo^bar')
+
+
+    def test_constrain_dependency_changed(self):
+        self.check_constrain_changed('libelf^foo', 'libelf^foo@1.0')
+        self.check_constrain_changed('libelf^foo', 'libelf^foo@1.0:5.0')
+        self.check_constrain_changed('libelf^foo', 'libelf^foo%gcc')
+        self.check_constrain_changed('libelf^foo%gcc', 'libelf^foo%gcc@4.5')
+        self.check_constrain_changed('libelf^foo', 'libelf^foo+debug')
+        self.check_constrain_changed('libelf^foo', 'libelf^foo~debug')
+        self.check_constrain_changed('libelf^foo', 'libelf^foo=bgqos_0')
+
+
+    def test_constrain_dependency_not_changed(self):
+        self.check_constrain_not_changed('libelf^foo@1.0', 'libelf^foo@1.0')
+        self.check_constrain_not_changed('libelf^foo@1.0:5.0', 'libelf^foo@1.0:5.0')
+        self.check_constrain_not_changed('libelf^foo%gcc', 'libelf^foo%gcc')
+        self.check_constrain_not_changed('libelf^foo%gcc@4.5', 'libelf^foo%gcc@4.5')
+        self.check_constrain_not_changed('libelf^foo+debug', 'libelf^foo+debug')
+        self.check_constrain_not_changed('libelf^foo~debug', 'libelf^foo~debug')
+        self.check_constrain_not_changed('libelf^foo=bgqos_0', 'libelf^foo=bgqos_0')
+
diff --git a/lib/spack/spack/test/spec_yaml.py b/lib/spack/spack/test/spec_yaml.py
new file mode 100644
index 0000000000..869befc02a
--- /dev/null
+++ b/lib/spack/spack/test/spec_yaml.py
@@ -0,0 +1,71 @@
+##############################################################################
+# Copyright (c) 2013-2015, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://scalability-llnl.github.io/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+"""Test YAML serialization for specs.
+
+YAML format preserves DAG informatoin in the spec.
+
+"""
+from spack.spec import Spec
+from spack.test.mock_packages_test import *
+
+class SpecDagTest(MockPackagesTest):
+
+    def check_yaml_round_trip(self, spec):
+        yaml_text = spec.to_yaml()
+        spec_from_yaml = Spec.from_yaml(yaml_text)
+        self.assertTrue(spec.eq_dag(spec_from_yaml))
+
+
+    def test_simple_spec(self):
+        spec = Spec('mpileaks')
+        self.check_yaml_round_trip(spec)
+
+
+    def test_normal_spec(self):
+        spec = Spec('mpileaks+debug~opt')
+        spec.normalize()
+        self.check_yaml_round_trip(spec)
+
+
+    def test_ambiguous_version_spec(self):
+        spec = Spec('mpileaks@1.0:5.0,6.1,7.3+debug~opt')
+        spec.normalize()
+        self.check_yaml_round_trip(spec)
+
+
+    def test_concrete_spec(self):
+        spec = Spec('mpileaks+debug~opt')
+        spec.concretize()
+        self.check_yaml_round_trip(spec)
+
+
+    def test_yaml_subdag(self):
+        spec = Spec('mpileaks^mpich+debug')
+        spec.concretize()
+
+        yaml_spec = Spec.from_yaml(spec.to_yaml())
+
+        for dep in ('callpath', 'mpich', 'dyninst', 'libdwarf', 'libelf'):
+            self.assertTrue(spec[dep].eq_dag(yaml_spec[dep]))
diff --git a/lib/spack/spack/util/debug.py b/lib/spack/spack/util/debug.py
new file mode 100644
index 0000000000..37985eccdd
--- /dev/null
+++ b/lib/spack/spack/util/debug.py
@@ -0,0 +1,52 @@
+##############################################################################
+# Copyright (c) 2013-2015, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://scalability-llnl.github.io/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+"""Debug signal handler: prints a stack trace and enters interpreter.
+
+``register_interrupt_handler()`` enables a ctrl-C handler that prints
+a stack trace and drops the user into an interpreter.
+
+"""
+import os
+import code
+import traceback
+import signal
+
+def debug_handler(sig, frame):
+    """Interrupt running process, and provide a python prompt for
+    interactive debugging."""
+    d = {'_frame':frame}         # Allow access to frame object.
+    d.update(frame.f_globals)    # Unless shadowed by global
+    d.update(frame.f_locals)
+
+    i = code.InteractiveConsole(d)
+    message  = "Signal received : entering python shell.\nTraceback:\n"
+    message += ''.join(traceback.format_stack(frame))
+    i.interact(message)
+    os._exit(1)  # Use os._exit to avoid test harness.
+
+
+def register_interrupt_handler():
+    """Register a handler to print a stack trace and enter an interpreter on Ctrl-C"""
+    signal.signal(signal.SIGINT, debug_handler)
diff --git a/lib/spack/spack/variant.py b/lib/spack/spack/variant.py
new file mode 100644
index 0000000000..3d3e2b0f6d
--- /dev/null
+++ b/lib/spack/spack/variant.py
@@ -0,0 +1,36 @@
+##############################################################################
+# Copyright (c) 2013, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://scalability-llnl.github.io/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+"""Variant is a class describing flags on builds, or "variants".
+
+Could be generalized later to describe aribitrary parameters, but
+currently variants are just flags.
+
+"""
+
+class Variant(object):
+    """Represents a variant on a build.  Can be either on or off."""
+    def __init__(self, default, description):
+        self.default     = bool(default)
+        self.description = str(description)
diff --git a/lib/spack/spack/version.py b/lib/spack/spack/version.py
index cc83634137..35db05e018 100644
--- a/lib/spack/spack/version.py
+++ b/lib/spack/spack/version.py
@@ -93,12 +93,12 @@ def coerce_versions(a, b):
 def coerced(method):
     """Decorator that ensures that argument types of a method are coerced."""
     @wraps(method)
-    def coercing_method(a, b):
+    def coercing_method(a, b, *args, **kwargs):
         if type(a) == type(b) or a is None or b is None:
-            return method(a, b)
+            return method(a, b, *args, **kwargs)
         else:
             ca, cb = coerce_versions(a, b)
-            return getattr(ca, method.__name__)(cb)
+            return getattr(ca, method.__name__)(cb, *args, **kwargs)
     return coercing_method
 
 
@@ -587,16 +587,42 @@ class VersionList(object):
         return False
 
 
+    def to_dict(self):
+        """Generate human-readable dict for YAML."""
+        if self.concrete:
+            return { 'version'  : str(self[0]) }
+        else:
+            return { 'versions' : [str(v) for v in self] }
+
+
+    @staticmethod
+    def from_dict(dictionary):
+        """Parse dict from to_dict."""
+        if 'versions' in dictionary:
+            return VersionList(dictionary['versions'])
+        elif 'version' in dictionary:
+            return VersionList([dictionary['version']])
+        else:
+            raise ValueError("Dict must have 'version' or 'versions' in it.")
+
+
     @coerced
-    def satisfies(self, other):
-        """A VersionList satisfies another if some version in the list would
-           would satisfy some version in the other list.  This uses essentially
-           the same algorithm as overlaps() does for VersionList, but it calls
-           satisfies() on member Versions and VersionRanges.
+    def satisfies(self, other, strict=False):
+        """A VersionList satisfies another if some version in the list
+           would satisfy some version in the other list.  This uses
+           essentially the same algorithm as overlaps() does for
+           VersionList, but it calls satisfies() on member Versions
+           and VersionRanges.
+
+           If strict is specified, this version list must lie entirely
+           *within* the other in order to satisfy it.
         """
         if not other or not self:
             return False
 
+        if strict:
+            return self in other
+
         s = o = 0
         while s < len(self) and o < len(other):
             if self[s].satisfies(other[o]):
@@ -633,9 +659,14 @@ class VersionList(object):
 
     @coerced
     def intersect(self, other):
+        """Intersect this spec's list with other.
+
+        Return True if the spec changed as a result; False otherwise
+        """
         isection = self.intersection(other)
+        changed = (isection.versions != self.versions)
         self.versions = isection.versions
-
+        return changed
 
     @coerced
     def __contains__(self, other):