diff --git a/lib/spack/spack/cmd/test.py b/lib/spack/spack/cmd/test.py
index c6ac064001..80eeff34ac 100644
--- a/lib/spack/spack/cmd/test.py
+++ b/lib/spack/spack/cmd/test.py
@@ -19,14 +19,13 @@ def setup_parser(subparser):
 
 def test(parser, args):
     if args.all:
-        for name in list_modules(spack.test_path):
+        for name in list_modules(spack.test_path, directories=False):
             print "Running Tests: %s" % name
             spack.test.run(name, verbose=args.verbose)
 
     elif not args.names:
         print "Available tests:"
-        colify(list_modules(spack.test_path))
-
+        colify(list_modules(spack.test_path, directories=False))
 
     else:
         for name in  args.names:
diff --git a/lib/spack/spack/compilers/__init__.py b/lib/spack/spack/compilers/__init__.py
index 24a99a1bae..ad2260d58a 100644
--- a/lib/spack/spack/compilers/__init__.py
+++ b/lib/spack/spack/compilers/__init__.py
@@ -11,5 +11,7 @@ def supported_compilers():
     return [c for c in list_modules(spack.compilers_path)]
 
 
-def get_compiler():
-    return Compiler('gcc', spack.compilers.gcc.get_version())
+@memoized
+def default_compiler():
+    from spack.spec import Compiler
+    return Compiler('gcc', gcc.get_version())
diff --git a/lib/spack/spack/concretize.py b/lib/spack/spack/concretize.py
new file mode 100644
index 0000000000..ac1d4421da
--- /dev/null
+++ b/lib/spack/spack/concretize.py
@@ -0,0 +1,80 @@
+"""
+Functions here are used to take abstract specs and make them concrete.
+For example, if a spec asks for a version between 1.8 and 1.9, these
+functions might take will take the most recent 1.9 version of the
+package available.  Or, if the user didn't specify a compiler for a
+spec, then this will assign a compiler to the spec based on defaults
+or user preferences.
+
+TODO: make this customizable and allow users to configure
+      concretization  policies.
+"""
+import spack.arch
+import spack.compilers
+from spack.version import *
+from spack.spec import *
+
+
+def concretize_version(spec):
+    """If the spec is already concrete, return.  Otherwise take
+       the most recent available version, and default to the package's
+       version if there are no avaialble versions.
+    """
+    # return if already concrete.
+    if spec.versions.concrete:
+        return
+
+    pkg = speck.package
+    available = pkg.available_versions
+
+    # If there are known avaialble versions, return the most recent
+    if versions:
+        spec.versions = ver([avaialble[-1]])
+    else:
+        spec.versions = ver([pkg.version])
+
+
+def concretize_architecture(spec):
+    """If the spec already had an architecture, return.  Otherwise if
+       the root of the DAG has an architecture, then use that.
+       Otherwise take the system's default architecture.
+
+       Intuition: Architectures won't be set a lot, and generally you
+       want the host system's architecture.  When architectures are
+       mised in a spec, it is likely because the tool requries a
+       cross-compiled component, e.g. for tools that run on BlueGene
+       or Cray machines.  These constraints will likely come directly
+       from packages, so require the user to be explicit if they want
+       to mess with the architecture, and revert to the default when
+       they're not explicit.
+    """
+    if spec.architecture is not None:
+        return
+
+    if spec.root.architecture:
+        spec.architecture = spec.root.architecture
+    else:
+        spec.architecture = spack.arch.sys_type()
+
+
+def concretize_compiler(spec):
+    """Currently just sets the compiler to gcc or throws an exception
+       if the compiler is set to something else.
+
+       TODO: implement below description.
+
+       If the spec already has a compiler, we're done.  If not, then
+       take the compiler used for the nearest ancestor with a concrete
+       compiler, or use the system default if there is no ancestor
+       with a compiler.
+
+       Intuition: Use the system default if no package that depends on
+       this one has a strict compiler requirement.  Otherwise, try to
+       build with the compiler that will be used by libraries that
+       link to this one, to maximize compatibility.
+    """
+    if spec.compiler.concrete:
+        if spec.compiler != spack.compilers.default_compiler():
+            raise spack.spec.UnknownCompilerError(str(spec.compiler))
+    else:
+        spec.compiler = spack.compilers.default_compiler()
diff --git a/lib/spack/spack/package.py b/lib/spack/spack/package.py
index 9eece8afcf..cd87adb990 100644
--- a/lib/spack/spack/package.py
+++ b/lib/spack/spack/package.py
@@ -18,13 +18,13 @@
 
 from spack import *
 import spack.spec
+import spack.error
 import packages
 import tty
 import attr
 import validate
 import url
 
-from spec import Compiler
 from version import *
 from multi_function import platform
 from stage import Stage
@@ -249,7 +249,7 @@ class SomePackage(Package):
     # These variables are per-package metadata will be defined by subclasses.
     #
     """By default a package has no dependencies."""
-    dependencies = []
+    dependencies = {}
 
     #
     # These are default values for instance variables.
@@ -371,21 +371,51 @@ def dependents(self):
         return tuple(self._dependents)
 
 
-    def sanity_check(self):
-        """Ensure that this package and its dependencies don't have conflicting
-           requirements."""
-        deps = sorted(self.all_dependencies, key=lambda d: d.name)
+    def preorder_traversal(self, visited=None):
+        if visited is None:
+            visited = set()
 
+        if self.name in visited:
+            return
+        visited.add(self.name)
+
+        yield self
+        for name, spec in self.dependencies.iteritems():
+            for pkg in packages.get(name).preorder_traversal(visited):
+                yield pkg
+
+
+    def validate_dependencies(self):
+        """Ensure that this package and its dependencies all have consistent
+           constraints on them.
+        """
+        # 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))
 
 
     @property
     @memoized
     def all_dependencies(self):
         """Dict(str -> Package) of all transitive dependencies of this package."""
-        all_deps = set(self.dependencies)
-        for dep in self.dependencies:
-            dep_pkg = packages.get(dep.name)
-            all_deps = all_deps.union(dep_pkg.all_dependencies)
+        all_deps = {name : dep for dep in self.preorder_traversal}
+        del all_deps[self.name]
         return all_deps
 
 
@@ -533,7 +563,7 @@ def setup_install_environment(self):
 
         # Pass along prefixes of dependencies here
         path_set(SPACK_DEPENDENCIES,
-                 [dep.package.prefix for dep in self.dependencies])
+                 [dep.package.prefix for dep in self.dependencies.values()])
 
         # Install location
         os.environ[SPACK_PREFIX] = self.prefix
@@ -544,7 +574,7 @@ def setup_install_environment(self):
 
     def do_install_dependencies(self):
         # Pass along paths of dependencies here
-        for dep in self.dependencies:
+        for dep in self.dependencies.values():
             dep.package.do_install()
 
 
@@ -607,7 +637,7 @@ def do_clean_dist(self):
     @property
     def available_versions(self):
         if not self._available_versions:
-            self._available_versions = VersionList()
+            self._available_versions = ver([self.version])
             try:
                 # Run curl but grab the mime type from the http headers
                 listing = spack.curl('-s', '-L', self.list_url, return_output=True)
@@ -617,18 +647,18 @@ def available_versions(self):
                 for s in strings:
                     match = re.search(wildcard, s)
                     if match:
-                        self._available_versions.add(ver(match.group(0)))
+                        self._available_versions.add(Version(match.group(0)))
 
-            except CalledProcessError:
+                if not self._available_versions:
+                    tty.warn("Found no versions for %s" % self.name,
+                             "Packate.available_versions may require adding the list_url attribute",
+                             "to the package to tell Spack where to look for versions.")
+
+            except subprocess.CalledProcessError:
                 tty.warn("Fetching %s failed." % self.list_url,
                          "Package.available_versions requires an internet connection.",
                          "Version list may be incomplete.")
 
-            if not self._available_versions:
-                tty.warn("Found no versions for %s" % self.name,
-                         "Packate.available_versions may require adding the list_url attribute",
-                         "to the package to tell Spack where to look for versions.")
-                self._available_versions = [self.version]
         return self._available_versions
 
 
@@ -654,3 +684,10 @@ def __call__(self, *args, **kwargs):
             args = (jobs,) + args
 
         super(MakeExecutable, self).__call__(*args, **kwargs)
+
+
+class InvalidPackageDependencyError(spack.error.SpackError):
+    """Raised when package specification is inconsistent with requirements of
+       its dependencies."""
+    def __init__(self, message):
+        super(InvalidPackageDependencyError, self).__init__(message)
diff --git a/lib/spack/spack/packages/__init__.py b/lib/spack/spack/packages/__init__.py
index 6d515274b6..8692dde5a8 100644
--- a/lib/spack/spack/packages/__init__.py
+++ b/lib/spack/spack/packages/__init__.py
@@ -20,6 +20,7 @@
 
 instances = {}
 
+
 def get(pkg_name):
     if not pkg_name in instances:
         package_class = get_class_for_package_name(pkg_name)
@@ -85,9 +86,18 @@ def get_class_for_package_name(pkg_name):
     else:
         raise UnknownPackageError(pkg_name)
 
+    # Figure out pacakges module from spack.packages_path
+    # This allows us to change the module path.
+    if not re.match(r'%s' % spack.module_path, spack.packages_path):
+        raise RuntimeError("Packages path is not a submodule of spack.")
+
+    # TODO: replace this with a proper package DB class, instead of this hackiness.
+    packages_path = re.sub(spack.module_path + '\/+', 'spack.', spack.packages_path)
+    packages_module = re.sub(r'\/', '.', packages_path)
+
     class_name = pkg_name.capitalize()
     try:
-        module_name = "%s.%s" % (__name__, pkg_name)
+        module_name = "%s.%s" % (packages_module, pkg_name)
         module = __import__(module_name, fromlist=[class_name])
     except ImportError, e:
         tty.die("Error while importing %s.%s:\n%s" % (pkg_name, class_name, e.message))
@@ -107,8 +117,8 @@ def compute_dependents():
         if pkg._dependents is None:
             pkg._dependents = []
 
-        for dep in pkg.dependencies:
-            dpkg = get(dep.name)
+        for name, dep in pkg.dependencies.iteritems():
+            dpkg = get(name)
             if dpkg._dependents is None:
                 dpkg._dependents = []
             dpkg._dependents.append(pkg.name)
@@ -130,8 +140,8 @@ def quote(string):
     deps = []
     for pkg in all_packages():
         out.write('  %-30s [label="%s"]\n' % (quote(pkg.name), pkg.name))
-        for dep in pkg.dependencies:
-            deps.append((pkg.name, dep.name))
+        for dep_name, dep in pkg.dependencies.iteritems():
+            deps.append((pkg.name, dep_name))
     out.write('\n')
 
     for pair in deps:
diff --git a/lib/spack/spack/packages/libdwarf.py b/lib/spack/spack/packages/libdwarf.py
index edaba6a216..bae701b38b 100644
--- a/lib/spack/spack/packages/libdwarf.py
+++ b/lib/spack/spack/packages/libdwarf.py
@@ -11,7 +11,7 @@ class Libdwarf(Package):
 
     list_url = "http://reality.sgiweb.org/davea/dwarf.html"
 
-    depends_on("libelf@0:1")
+    depends_on("libelf")
 
 
     def clean(self):
diff --git a/lib/spack/spack/parse.py b/lib/spack/spack/parse.py
index 5431aa454d..f2381b1078 100644
--- a/lib/spack/spack/parse.py
+++ b/lib/spack/spack/parse.py
@@ -91,12 +91,16 @@ def expect(self, id):
                 self.next_token_error("Unexpected end of input")
             sys.exit(1)
 
-    def parse(self, text):
+    def setup(self, text):
         self.text = text
         self.push_tokens(self.lexer.lex(text))
+
+    def parse(self, text):
+        self.setup(text)
         return self.do_parse()
 
 
+
 class ParseError(spack.error.SpackError):
     """Raised when we don't hit an error while parsing."""
     def __init__(self, message, string, pos):
diff --git a/lib/spack/spack/relations.py b/lib/spack/spack/relations.py
index 8eb4e2d1a2..cc690ffc0f 100644
--- a/lib/spack/spack/relations.py
+++ b/lib/spack/spack/relations.py
@@ -54,10 +54,10 @@ def depends_on(*specs):
     """
     # Get the enclosing package's scope and add deps to it.
     locals = sys._getframe(1).f_locals
-    dependencies = locals.setdefault("dependencies", [])
+    dependencies = locals.setdefault("dependencies", {})
     for string in specs:
         for spec in spack.spec.parse(string):
-            dependencies.append(spec)
+            dependencies[spec.name] = spec
 
 
 def provides(*args):
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index edc97c7c3b..2731c344ab 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -67,6 +67,7 @@
 import tty
 import spack.parse
 import spack.error
+import spack.concretize
 import spack.compilers
 import spack.compilers.gcc
 import spack.packages as packages
@@ -137,9 +138,8 @@ def satisfies(self, other):
 
 
     def constrain(self, other):
-        if not self.satisfies(other.compiler):
-            raise UnsatisfiableCompilerSpecError(
-                "%s does not satisfy %s" % (self.compiler, other.compiler))
+        if not self.satisfies(other):
+            raise UnsatisfiableCompilerSpecError(self, other)
 
         self.versions.intersect(other.versions)
 
@@ -149,23 +149,6 @@ def concrete(self):
         return self.versions.concrete
 
 
-    def _concretize(self):
-        """If this spec could describe more than one version, variant, or build
-           of a package, this will resolve it to be concrete.
-        """
-        # TODO: support compilers other than GCC.
-        if self.concrete:
-            return
-        gcc_version = spack.compilers.gcc.get_version()
-        self.versions = VersionList([gcc_version])
-
-
-    def concretized(self):
-        clone = self.copy()
-        clone._concretize()
-        return clone
-
-
     @property
     def version(self):
         if not self.concrete:
@@ -243,13 +226,34 @@ def __str__(self):
 
 @key_ordering
 class Spec(object):
-    def __init__(self, name):
-        self.name = name
-        self.versions = VersionList()
-        self.variants = VariantMap()
-        self.architecture = None
-        self.compiler = None
-        self.dependencies = DependencyMap()
+    def __init__(self, spec_like):
+        # Copy if spec_like is a Spec.
+        if type(spec_like) == Spec:
+            self._dup(spec_like)
+            return
+
+        # Parse if the spec_like is a string.
+        if type(spec_like) != str:
+            raise TypeError("Can't make spec out of %s" % type(spec_like))
+
+        spec_list = SpecParser().parse(spec_like)
+        if len(spec_list) > 1:
+            raise ValueError("More than one spec in string: " + spec_like)
+        if len(spec_list) < 1:
+            raise ValueError("String contains no specs: " + spec_like)
+
+        # Take all the attributes from the first parsed spec without copying
+        # This is a little bit nasty, but it's nastier to make the parser
+        # write directly into this Spec object.
+        other = spec_list[0]
+        self.name = other.name
+        self.parent = other.parent
+        self.versions = other.versions
+        self.variants = other.variants
+        self.architecture = other.architecture
+        self.compiler = other.compiler
+        self.dependencies = other.dependencies
+
 
     #
     # Private routines here are called by the parser when building a spec.
@@ -285,6 +289,21 @@ def _add_dependency(self, dep):
         if dep.name in self.dependencies:
             raise DuplicateDependencyError("Cannot depend on '%s' twice" % dep)
         self.dependencies[dep.name] = dep
+        dep.parent = self
+
+
+    @property
+    def root(self):
+        """Follow parent links and find the root of this spec's DAG."""
+        root = self
+        while root.parent is not None:
+            root = root.parent
+        return root
+
+
+    @property
+    def package(self):
+        return packages.get(self.name)
 
 
     @property
@@ -296,6 +315,20 @@ def concrete(self):
                     and self.dependencies.concrete)
 
 
+    def preorder_traversal(self, visited=None):
+        if visited is None:
+            visited = set()
+
+        if id(self) in visited:
+            return
+        visited.add(id(self))
+
+        yield self
+        for dep in self.dependencies.itervalues():
+            for spec in dep.preorder_traversal(visited):
+                yield spec
+
+
     def _concretize(self):
         """A spec is concrete if it describes one build of a package uniquely.
            This will ensure that this spec is concrete.
@@ -327,30 +360,40 @@ def _concretize(self):
 
         # TODO: handle variants.
 
-        pkg = packages.get(self.name)
-
         # Take the highest version in a range
         if not self.versions.concrete:
-            preferred = self.versions.highest() or pkg.version
+            preferred = self.versions.highest() or self.package.version
             self.versions = VersionList([preferred])
 
         # Ensure dependencies have right versions
 
 
-    @property
-    def traverse_deps(self, visited=None):
-        """Yields dependencies in depth-first order"""
-        if not visited:
-            visited = set()
+    def flatten(self):
+        """Pull all dependencies up to the root (this spec).
+           Merge constraints for dependencies with the same name, and if they
+           conflict, throw an exception. """
+        # This ensures that the package descriptions themselves are consistent
+        self.package.validate_dependencies()
 
-        for name in sorted(self.dependencies.keys()):
-            dep = dependencies[name]
-            if dep in visited:
-                continue
+        # Once that is guaranteed, we know any constraint violations are due
+        # to the spec -- so they're the user's fault, not Spack's.
+        flat_deps = DependencyMap()
+        try:
+            for spec in self.preorder_traversal():
+                if spec.name not in flat_deps:
+                    flat_deps[spec.name] = spec
+                else:
+                    flat_deps[spec.name].constrain(spec)
 
-            for d in dep.traverse_deps(seen):
-                yield d
-            yield dep
+        except UnsatisfiableSpecError, e:
+            # This REALLY shouldn't happen unless something is wrong in spack.
+            # It means we got a spec DAG with two instances of the same package
+            # that had inconsistent constraints.  There's no way for a user to
+            # produce a spec like this (the parser adds all deps to the root),
+            # so this means OUR code is not sane!
+            raise InconsistentSpecError("Invalid Spec DAG: %s" % e.message)
+
+        self.dependencies = flat_deps
 
 
     def _normalize_helper(self, visited, spec_deps):
@@ -362,9 +405,7 @@ def _normalize_helper(self, visited, spec_deps):
         # Combine constraints from package dependencies with
         # information in this spec's dependencies.
         pkg = packages.get(self.name)
-        for pkg_dep in pkg.dependencies:
-            name = pkg_dep.name
-
+        for name, pkg_dep in self.package.dependencies.iteritems():
             if name not in spec_deps:
                 # Clone the spec from the package
                 spec_deps[name] = pkg_dep.copy()
@@ -372,23 +413,29 @@ def _normalize_helper(self, visited, spec_deps):
             try:
                 # intersect package information with spec info
                 spec_deps[name].constrain(pkg_dep)
+
             except UnsatisfiableSpecError, e:
-                error_type = type(e)
-                raise error_type(
-                    "Violated depends_on constraint from package %s: %s"
-                    % (self.name, e.message))
+                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
-            self.dependencies[name] = spec_deps[name]
+            self._add_dependency(spec_deps[name])
             self.dependencies[name]._normalize_helper(visited, spec_deps)
 
 
     def normalize(self):
-        if any(dep.dependencies for dep in self.dependencies.values()):
-            raise SpecError("Spec has already been normalized.")
-
+        # Ensure first that all packages exist.
         self.validate_package_names()
 
+        # Then ensure that the packages mentioned are sane, that the
+        # provided spec is sane, and that all dependency specs are in the
+        # root node of the spec.  Flatten will do this for us.
+        self.flatten()
+
+        # Now that we're flat we can get all our dependencies at once.
         spec_deps = self.dependencies
         self.dependencies = DependencyMap()
 
@@ -404,29 +451,25 @@ def normalize(self):
 
 
     def validate_package_names(self):
-        for name in self.dependencies:
-            packages.get(name)
+        packages.get(self.name)
+        for name, dep in self.dependencies.iteritems():
+            dep.validate_package_names()
 
 
     def constrain(self, other):
         if not self.versions.overlaps(other.versions):
-            raise UnsatisfiableVersionSpecError(
-                "%s does not satisfy %s" % (self.versions, other.versions))
+            raise UnsatisfiableVersionSpecError(self.versions, other.versions)
 
-        conflicting_variants = [
-            v for v in other.variants if v in self.variants and
-            self.variants[v].enabled != other.variants[v].enabled]
-
-        if conflicting_variants:
-            raise UnsatisfiableVariantSpecError(comma_and(
-                "%s does not satisfy %s" % (self.variants[v], other.variants[v])
-                for v in conflicting_variants))
+        for v in other.variants:
+            if (v in self.variants and
+                self.variants[v].enabled != other.variants[v].enabled):
+                raise UnsatisfiableVariantSpecError(self.variants[v],
+                                                    other.variants[v])
 
         if self.architecture is not None and other.architecture is not None:
             if self.architecture != other.architecture:
-                raise UnsatisfiableArchitectureSpecError(
-                    "Asked for architecture %s, but required %s"
-                    % (self.architecture, other.architecture))
+                raise UnsatisfiableArchitectureSpecError(self.architecture,
+                                                         other.architecture)
 
         if self.compiler is not None and other.compiler is not None:
             self.compiler.constrain(other.compiler)
@@ -457,16 +500,23 @@ def concretized(self):
         return clone
 
 
+    def _dup(self, other):
+        """Copy the spec other into self.  This is a
+           first-party, overwriting copy."""
+        # TODO: this needs to handle DAGs.
+        self.name = other.name
+        self.versions = other.versions.copy()
+        self.variants = other.variants.copy()
+        self.architecture = other.architecture
+        self.compiler = None
+        if other.compiler:
+            self.compiler = other.compiler.copy()
+        self.dependencies = other.dependencies.copy()
+
+
     def copy(self):
-        clone = Spec(self.name)
-        clone.versions = self.versions.copy()
-        clone.variants = self.variants.copy()
-        clone.architecture = self.architecture
-        clone.compiler = None
-        if self.compiler:
-            clone.compiler = self.compiler.copy()
-        clone.dependencies = self.dependencies.copy()
-        return clone
+        """Return a deep copy of this spec."""
+        return Spec(self)
 
 
     @property
@@ -478,7 +528,7 @@ def version(self):
 
     def _cmp_key(self):
         return (self.name, self.versions, self.variants,
-                self.architecture, self.compiler)
+                self.architecture, self.compiler, self.dependencies)
 
 
     def colorized(self):
@@ -505,7 +555,7 @@ def str_without_deps(self):
 
     def tree(self, indent=""):
         """Prints out this spec and its dependencies, tree-formatted
-           with indentation."""
+           with indentation.  Each node also has an id."""
         out = indent + self.str_without_deps()
         for dep in sorted(self.dependencies.keys()):
             out += "\n" + self.dependencies[dep].tree(indent + "    ")
@@ -566,8 +616,22 @@ def do_parse(self):
 
 
     def spec(self):
+        """Parse a spec out of the input.  If a spec is supplied, then initialize
+           and return it instead of creating a new one."""
         self.check_identifier()
-        spec = Spec(self.token.value)
+
+        # This will init the spec without calling __init__.
+        spec = Spec.__new__(Spec)
+        spec.name = self.token.value
+        spec.parent = None
+        spec.versions = VersionList()
+        spec.variants = VariantMap()
+        spec.architecture = None
+        spec.compiler = None
+        spec.dependencies = DependencyMap()
+
+        # record this so that we know whether version is
+        # unspecified or not.
         added_version = False
 
         while self.next:
@@ -661,34 +725,10 @@ def check_identifier(self):
 
 
 def parse(string):
-    """Returns a list of specs from an input string."""
-    return SpecParser().parse(string)
-
-
-def parse_one(string):
-    """Parses a string containing only one spec, then returns that
-       spec.  If more than one spec is found, raises a ValueError.
+    """Returns a list of specs from an input string.
+       For creating one spec, see Spec() constructor.
     """
-    spec_list = parse(string)
-    if len(spec_list) > 1:
-        raise ValueError("string contains more than one spec!")
-    elif len(spec_list) < 1:
-        raise ValueError("string contains no specs!")
-    return spec_list[0]
-
-
-def make_spec(spec_like):
-    if type(spec_like) == str:
-        specs = parse(spec_like)
-        if len(specs) != 1:
-            raise ValueError("String contains multiple specs: '%s'" % spec_like)
-        return specs[0]
-
-    elif type(spec_like) == Spec:
-        return spec_like
-
-    else:
-        raise TypeError("Can't make spec out of %s" % type(spec_like))
+    return SpecParser().parse(string)
 
 
 class SpecError(spack.error.SpackError):
@@ -728,6 +768,13 @@ def __init__(self, message):
         super(DuplicateArchitectureError, self).__init__(message)
 
 
+class InconsistentSpecError(SpecError):
+    """Raised when two nodes in the same spec DAG have inconsistent
+       constraints."""
+    def __init__(self, message):
+        super(InconsistentSpecError, self).__init__(message)
+
+
 class InvalidDependencyException(SpecError):
     """Raised when a dependency in a spec is not actually a dependency
        of the package."""
@@ -736,30 +783,39 @@ def __init__(self, message):
 
 
 class UnsatisfiableSpecError(SpecError):
-    """Raised when a spec conflicts with package constraints."""
-    def __init__(self, message):
-        super(UnsatisfiableSpecError, self).__init__(message)
+    """Raised when a spec conflicts with package constraints.
+       Provide the requirement that was violated when raising."""
+    def __init__(self, provided, required, constraint_type):
+        super(UnsatisfiableSpecError, self).__init__(
+            "%s does not satisfy %s" % (provided, required))
+        self.provided = provided
+        self.required = required
+        self.constraint_type = constraint_type
 
 
 class UnsatisfiableVersionSpecError(UnsatisfiableSpecError):
     """Raised when a spec version conflicts with package constraints."""
-    def __init__(self, message):
-        super(UnsatisfiableVersionSpecError, self).__init__(message)
+    def __init__(self, provided, required):
+        super(UnsatisfiableVersionSpecError, self).__init__(
+            provided, required, "version")
 
 
 class UnsatisfiableCompilerSpecError(UnsatisfiableSpecError):
     """Raised when a spec comiler conflicts with package constraints."""
-    def __init__(self, message):
-        super(UnsatisfiableCompilerSpecError, self).__init__(message)
+    def __init__(self, provided, required):
+        super(UnsatisfiableCompilerSpecError, self).__init__(
+            provided, required, "compiler")
 
 
 class UnsatisfiableVariantSpecError(UnsatisfiableSpecError):
     """Raised when a spec variant conflicts with package constraints."""
-    def __init__(self, message):
-        super(UnsatisfiableVariantSpecError, self).__init__(message)
+    def __init__(self, provided, required):
+        super(UnsatisfiableVariantSpecError, self).__init__(
+            provided, required, "variant")
 
 
 class UnsatisfiableArchitectureSpecError(UnsatisfiableSpecError):
     """Raised when a spec architecture conflicts with package constraints."""
-    def __init__(self, message):
-        super(UnsatisfiableArchitectureSpecError, self).__init__(message)
+    def __init__(self, provided, required):
+        super(UnsatisfiableArchitectureSpecError, self).__init__(
+            provided, required, "architecture")
diff --git a/lib/spack/spack/test/concretize.py b/lib/spack/spack/test/concretize.py
index 3a528f1b16..f010d09c0e 100644
--- a/lib/spack/spack/test/concretize.py
+++ b/lib/spack/spack/test/concretize.py
@@ -1,11 +1,11 @@
 import unittest
-import spack.spec
+from spack.spec import Spec
 
 
 class ConcretizeTest(unittest.TestCase):
 
     def check_concretize(self, abstract_spec):
-        abstract = spack.spec.parse_one(abstract_spec)
+        abstract = Spec(abstract_spec)
         print abstract
         print abstract.concretized()
         print abstract.concretized().concrete
diff --git a/lib/spack/spack/test/mock_packages/__init__.py b/lib/spack/spack/test/mock_packages/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/lib/spack/spack/test/mock_packages/callpath.py b/lib/spack/spack/test/mock_packages/callpath.py
new file mode 100644
index 0000000000..958960e0ab
--- /dev/null
+++ b/lib/spack/spack/test/mock_packages/callpath.py
@@ -0,0 +1,14 @@
+from spack import *
+
+class Callpath(Package):
+    homepage = "https://github.com/tgamblin/callpath"
+    url      = "http://github.com/tgamblin/callpath-0.2.tar.gz"
+    md5      = "foobarbaz"
+
+    depends_on("dyninst")
+    depends_on("mpich")
+
+    def install(self, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")
diff --git a/lib/spack/spack/test/mock_packages/dyninst.py b/lib/spack/spack/test/mock_packages/dyninst.py
new file mode 100644
index 0000000000..f550cde54f
--- /dev/null
+++ b/lib/spack/spack/test/mock_packages/dyninst.py
@@ -0,0 +1,14 @@
+from spack import *
+
+class Dyninst(Package):
+    homepage = "https://paradyn.org"
+    url      = "http://www.dyninst.org/sites/default/files/downloads/dyninst/8.1.2/DyninstAPI-8.1.2.tgz"
+    md5      = "bf03b33375afa66fe0efa46ce3f4b17a"
+
+    depends_on("libelf")
+    depends_on("libdwarf")
+
+    def install(self, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")
diff --git a/lib/spack/spack/test/mock_packages/libdwarf.py b/lib/spack/spack/test/mock_packages/libdwarf.py
new file mode 100644
index 0000000000..bae701b38b
--- /dev/null
+++ b/lib/spack/spack/test/mock_packages/libdwarf.py
@@ -0,0 +1,55 @@
+from spack import *
+import os
+
+# Only build certain parts of dwarf because the other ones break.
+dwarf_dirs = ['libdwarf', 'dwarfdump2']
+
+class Libdwarf(Package):
+    homepage = "http://reality.sgiweb.org/davea/dwarf.html"
+    url      = "http://reality.sgiweb.org/davea/libdwarf-20130207.tar.gz"
+    md5      = "64b42692e947d5180e162e46c689dfbf"
+
+    list_url = "http://reality.sgiweb.org/davea/dwarf.html"
+
+    depends_on("libelf")
+
+
+    def clean(self):
+        for dir in dwarf_dirs:
+            with working_dir(dir):
+                if os.path.exists('Makefile'):
+                    make('clean')
+
+
+    def install(self, prefix):
+        # dwarf build does not set arguments for ar properly
+        make.add_default_arg('ARFLAGS=rcs')
+
+        # Dwarf doesn't provide an install, so we have to do it.
+        mkdirp(bin, include, lib, man1)
+
+        with working_dir('libdwarf'):
+            configure("--prefix=%s" % prefix, '--enable-shared')
+            make()
+
+            install('libdwarf.a',  lib)
+            install('libdwarf.so', lib)
+            install('libdwarf.h',  include)
+            install('dwarf.h',     include)
+
+        with working_dir('dwarfdump2'):
+            configure("--prefix=%s" % prefix)
+
+            # This makefile has strings of copy commands that
+            # cause a race in parallel
+            make(parallel=False)
+
+            install('dwarfdump',     bin)
+            install('dwarfdump.conf', lib)
+            install('dwarfdump.1',    man1)
+
+
+    @platform('macosx_10.8_x86_64')
+    def install(self, prefix):
+        raise UnsupportedPlatformError(
+            "libdwarf doesn't currently build on Mac OS X.")
diff --git a/lib/spack/spack/test/mock_packages/libelf.py b/lib/spack/spack/test/mock_packages/libelf.py
new file mode 100644
index 0000000000..7e3046b174
--- /dev/null
+++ b/lib/spack/spack/test/mock_packages/libelf.py
@@ -0,0 +1,16 @@
+from spack import *
+
+class Libelf(Package):
+    homepage = "http://www.mr511.de/software/english.html"
+    url      = "http://www.mr511.de/software/libelf-0.8.13.tar.gz"
+    md5      = "4136d7b4c04df68b686570afa26988ac"
+
+    def install(self, prefix):
+        configure("--prefix=%s" % prefix,
+                  "--enable-shared",
+                  "--disable-dependency-tracking",
+                  "--disable-debug")
+        make()
+
+        # The mkdir commands in libelf's intsall can fail in parallel
+        make("install", parallel=False)
diff --git a/lib/spack/spack/test/mock_packages/mpich.py b/lib/spack/spack/test/mock_packages/mpich.py
new file mode 100644
index 0000000000..d8cd67d528
--- /dev/null
+++ b/lib/spack/spack/test/mock_packages/mpich.py
@@ -0,0 +1,11 @@
+from spack import *
+
+class Mpich(Package):
+    homepage = "http://www.mpich.org"
+    url      = "http://www.mpich.org/static/downloads/3.0.4/mpich-3.0.4.tar.gz"
+    md5      = "9c5d5d4fe1e17dd12153f40bc5b6dbc0"
+
+    def install(self, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")
diff --git a/lib/spack/spack/test/mock_packages/mpileaks.py b/lib/spack/spack/test/mock_packages/mpileaks.py
new file mode 100644
index 0000000000..224557cc52
--- /dev/null
+++ b/lib/spack/spack/test/mock_packages/mpileaks.py
@@ -0,0 +1,14 @@
+from spack import *
+
+class Mpileaks(Package):
+    homepage = "http://www.llnl.gov"
+    url      = "http://www.llnl.gov/mpileaks-1.0.tar.gz"
+    md5      = "foobarbaz"
+
+    depends_on("mpich")
+    depends_on("callpath")
+
+    def install(self, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")
diff --git a/lib/spack/spack/test/spec_dag.py b/lib/spack/spack/test/spec_dag.py
new file mode 100644
index 0000000000..a9dd0ec41c
--- /dev/null
+++ b/lib/spack/spack/test/spec_dag.py
@@ -0,0 +1,117 @@
+"""
+These tests check validation of dummy packages.  You can find the dummy
+packages directories that these tests use in:
+
+    spack/lib/spack/spack/test/mock_packages
+
+Each test validates conditions with the packages in those directories.
+"""
+import unittest
+
+import spack
+import spack.package
+import spack.packages as packages
+
+from spack.util.lang import new_path, list_modules
+from spack.spec import Spec
+
+mock_packages_path = new_path(spack.module_path, 'test', 'mock_packages')
+
+
+def set_pkg_dep(pkg, spec):
+    """Alters dependence information for a pacakge.
+       Use this to mock up constraints.
+    """
+    spec = Spec(spec)
+    packages.get(pkg).dependencies[spec.name] = spec
+
+
+class ValidationTest(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        # Use a different packages directory for these tests.  We want to use
+        # mocked up packages that don't interfere with the real ones.
+        cls.real_packages_path = spack.packages_path
+        spack.packages_path = mock_packages_path
+
+        # First time through, record original relationships bt/w packages
+        cls.original_deps = {}
+        for name in list_modules(mock_packages_path):
+            pkg = packages.get(name)
+            cls.original_deps[name] = [
+                spec for spec in pkg.dependencies.values()]
+
+
+    @classmethod
+    def restore(cls):
+        # each time through restore original dependencies & constraints
+        for pkg_name, deps in cls.original_deps.iteritems():
+            packages.get(pkg_name).dependencies.clear()
+            for dep in deps:
+                set_pkg_dep(pkg_name, dep)
+
+    @classmethod
+    def tearDownClass(cls):
+        """Restore the real packages path after any test."""
+        cls.restore()
+        spack.packages_path = cls.real_packages_path
+
+
+    def setUp(self):
+        """Before each test, restore deps between packages to original state."""
+        ValidationTest.restore()
+
+
+    def test_conflicting_package_constraints(self):
+        set_pkg_dep('mpileaks', 'mpich@1.0')
+        set_pkg_dep('callpath', 'mpich@2.0')
+
+        spec = Spec('mpileaks ^mpich ^callpath ^dyninst ^libelf ^libdwarf')
+        self.assertRaises(spack.package.InvalidPackageDependencyError,
+                          spec.package.validate_dependencies)
+
+
+    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)
+        mpileaks.normalize()
+        mpileaks.dependencies['mpich'] = Spec('mpich@1.0')
+        mpileaks.dependencies['callpath'].dependencies['mpich'] = Spec('mpich@2.0')
+
+        self.assertRaises(spack.spec.InconsistentSpecError, mpileaks.flatten)
+
+
+    def test_unsatisfiable_version(self):
+        set_pkg_dep('mpileaks', 'mpich@1.0')
+        spec = Spec('mpileaks ^mpich@2.0 ^callpath ^dyninst ^libelf ^libdwarf')
+        self.assertRaises(spack.spec.UnsatisfiableVersionSpecError, spec.normalize)
+
+
+    def test_unsatisfiable_compiler(self):
+        set_pkg_dep('mpileaks', 'mpich%gcc')
+        spec = Spec('mpileaks ^mpich%intel ^callpath ^dyninst ^libelf ^libdwarf')
+        self.assertRaises(spack.spec.UnsatisfiableCompilerSpecError, spec.normalize)
+
+
+    def test_unsatisfiable_compiler_version(self):
+        set_pkg_dep('mpileaks', 'mpich%gcc@4.6')
+        spec = Spec('mpileaks ^mpich%gcc@4.5 ^callpath ^dyninst ^libelf ^libdwarf')
+        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')
+        self.assertRaises(spack.spec.UnsatisfiableArchitectureSpecError, spec.normalize)
diff --git a/lib/spack/spack/test/specs.py b/lib/spack/spack/test/spec_syntax.py
similarity index 78%
rename from lib/spack/spack/test/specs.py
rename to lib/spack/spack/test/spec_syntax.py
index cb8bf79ff8..33534a4c1d 100644
--- a/lib/spack/spack/test/specs.py
+++ b/lib/spack/spack/test/spec_syntax.py
@@ -61,23 +61,40 @@ def check_lex(self, tokens, spec):
 
 
     def check_satisfies(self, lspec, rspec):
-        l = spack.spec.parse_one(lspec)
-        r = spack.spec.parse_one(rspec)
-        self.assertTrue(l.satisfies(r) and r.satisfies(l))
+        l, r = Spec(lspec), Spec(rspec)
+        self.assertTrue(l.satisfies(r))
+        self.assertTrue(r.satisfies(l))
 
-        # These should not raise
-        l.constrain(r)
-        r.constrain(l)
+        try:
+            l.constrain(r)
+            r.constrain(l)
+        except SpecError, e:
+            self.fail("Got a SpecError in constrain!", e.message)
+
+
+    def assert_unsatisfiable(lspec, rspec):
+        l, r = Spec(lspec), Spec(rspec)
+        self.assertFalse(l.satisfies(r))
+        self.assertFalse(r.satisfies(l))
+
+        self.assertRaises(l.constrain, r)
+        self.assertRaises(r.constrain, l)
 
 
     def check_constrain(self, expected, constrained, constraint):
-        exp = spack.spec.parse_one(expected)
-        constrained = spack.spec.parse_one(constrained)
-        constraint = spack.spec.parse_one(constraint)
+        exp = Spec(expected)
+        constrained = Spec(constrained)
+        constraint = Spec(constraint)
         constrained.constrain(constraint)
         self.assertEqual(exp, constrained)
 
 
+    def check_invalid_constraint(self, constrained, constraint):
+        constrained = Spec(constrained)
+        constraint = Spec(constraint)
+        self.assertRaises(UnsatisfiableSpecError, constrained.constrain, constraint)
+
+
     # ================================================================================
     # Parse checks
     # ===============================================================================
@@ -145,7 +162,28 @@ def test_satisfies(self):
 
 
     def test_constrain(self):
-        self.check_constrain('libelf@0:1', 'libelf', 'libelf@0:1')
+        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')
+
+        self.check_constrain('libelf+debug+foo', 'libelf+debug', 'libelf+foo')
+        self.check_constrain('libelf+debug+foo', 'libelf+debug', 'libelf+debug+foo')
+
+        self.check_constrain('libelf+debug~foo', 'libelf+debug', 'libelf~foo')
+        self.check_constrain('libelf+debug~foo', 'libelf+debug', 'libelf+debug~foo')
+
+        self.check_constrain('libelf=bgqos_0', 'libelf=bgqos_0', 'libelf=bgqos_0')
+        self.check_constrain('libelf=bgqos_0', 'libelf', 'libelf=bgqos_0')
+
+
+    def test_invalid_constraint(self):
+        self.check_invalid_constraint('libelf@0:2.0', 'libelf@2.1:3')
+        self.check_invalid_constraint('libelf@0:2.5%gcc@4.8:4.9', 'libelf@2.1:3%gcc@4.5:4.7')
+
+        self.check_invalid_constraint('libelf+debug', 'libelf~debug')
+        self.check_invalid_constraint('libelf+debug~foo', 'libelf+debug+foo')
+
+        self.check_invalid_constraint('libelf=bgqos_0', 'libelf=x86_54')
 
 
     # ================================================================================
diff --git a/lib/spack/spack/util/executable.py b/lib/spack/spack/util/executable.py
index 99b52ea299..d44420ab72 100644
--- a/lib/spack/spack/util/executable.py
+++ b/lib/spack/spack/util/executable.py
@@ -1,4 +1,5 @@
 import os
+import re
 import subprocess
 import spack.tty as tty
 
diff --git a/lib/spack/spack/util/lang.py b/lib/spack/spack/util/lang.py
index 92532c109f..90546c3f2f 100644
--- a/lib/spack/spack/util/lang.py
+++ b/lib/spack/spack/util/lang.py
@@ -5,6 +5,9 @@
 import inspect
 from spack.util.filesystem import new_path
 
+# Ignore emacs backups when listing modules
+ignore_modules = [r'^\.#', '~$']
+
 
 def has_method(cls, name):
     for base in inspect.getmro(cls):
@@ -27,21 +30,24 @@ def memoizer(*args, **kwargs):
     return memoizer
 
 
-def list_modules(directory):
+def list_modules(directory, **kwargs):
     """Lists all of the modules, excluding __init__.py, in
        a particular directory."""
+    list_directories = kwargs.setdefault('directories', True)
+
     for name in os.listdir(directory):
         if name == '__init__.py':
             continue
 
         path = new_path(directory, name)
-        if os.path.isdir(path):
+        if list_directories and os.path.isdir(path):
             init_py = new_path(path, '__init__.py')
             if os.path.isfile(init_py):
                 yield name
 
         elif name.endswith('.py'):
-            yield re.sub('.py$', '', name)
+            if not any(re.search(pattern, name) for pattern in ignore_modules):
+                yield re.sub('.py$', '', name)
 
 
 def key_ordering(cls):