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diff --git a/engine/SCons/compat/_scons_sets.py b/engine/SCons/compat/_scons_sets.py
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+"""Classes to represent arbitrary sets (including sets of sets).
+
+This module implements sets using dictionaries whose values are
+ignored.  The usual operations (union, intersection, deletion, etc.)
+are provided as both methods and operators.
+
+Important: sets are not sequences!  While they support 'x in s',
+'len(s)', and 'for x in s', none of those operations are unique for
+sequences; for example, mappings support all three as well.  The
+characteristic operation for sequences is subscripting with small
+integers: s[i], for i in range(len(s)).  Sets don't support
+subscripting at all.  Also, sequences allow multiple occurrences and
+their elements have a definite order; sets on the other hand don't
+record multiple occurrences and don't remember the order of element
+insertion (which is why they don't support s[i]).
+
+The following classes are provided:
+
+BaseSet -- All the operations common to both mutable and immutable
+    sets. This is an abstract class, not meant to be directly
+    instantiated.
+
+Set -- Mutable sets, subclass of BaseSet; not hashable.
+
+ImmutableSet -- Immutable sets, subclass of BaseSet; hashable.
+    An iterable argument is mandatory to create an ImmutableSet.
+
+_TemporarilyImmutableSet -- A wrapper around a Set, hashable,
+    giving the same hash value as the immutable set equivalent
+    would have.  Do not use this class directly.
+
+Only hashable objects can be added to a Set. In particular, you cannot
+really add a Set as an element to another Set; if you try, what is
+actually added is an ImmutableSet built from it (it compares equal to
+the one you tried adding).
+
+When you ask if `x in y' where x is a Set and y is a Set or
+ImmutableSet, x is wrapped into a _TemporarilyImmutableSet z, and
+what's tested is actually `z in y'.
+
+"""
+
+# Code history:
+#
+# - Greg V. Wilson wrote the first version, using a different approach
+#   to the mutable/immutable problem, and inheriting from dict.
+#
+# - Alex Martelli modified Greg's version to implement the current
+#   Set/ImmutableSet approach, and make the data an attribute.
+#
+# - Guido van Rossum rewrote much of the code, made some API changes,
+#   and cleaned up the docstrings.
+#
+# - Raymond Hettinger added a number of speedups and other
+#   improvements.
+
+from __future__ import generators
+try:
+    from itertools import ifilter, ifilterfalse
+except ImportError:
+    # Code to make the module run under Py2.2
+    def ifilter(predicate, iterable):
+        if predicate is None:
+            def predicate(x):
+                return x
+        for x in iterable:
+            if predicate(x):
+                yield x
+    def ifilterfalse(predicate, iterable):
+        if predicate is None:
+            def predicate(x):
+                return x
+        for x in iterable:
+            if not predicate(x):
+                yield x
+    try:
+        True, False
+    except NameError:
+        True, False = (0==0, 0!=0)
+
+__all__ = ['BaseSet', 'Set', 'ImmutableSet']
+
+class BaseSet(object):
+    """Common base class for mutable and immutable sets."""
+
+    __slots__ = ['_data']
+
+    # Constructor
+
+    def __init__(self):
+        """This is an abstract class."""
+        # Don't call this from a concrete subclass!
+        if self.__class__ is BaseSet:
+            raise TypeError, ("BaseSet is an abstract class.  "
+                              "Use Set or ImmutableSet.")
+
+    # Standard protocols: __len__, __repr__, __str__, __iter__
+
+    def __len__(self):
+        """Return the number of elements of a set."""
+        return len(self._data)
+
+    def __repr__(self):
+        """Return string representation of a set.
+
+        This looks like 'Set([<list of elements>])'.
+        """
+        return self._repr()
+
+    # __str__ is the same as __repr__
+    __str__ = __repr__
+
+    def _repr(self, sorted=False):
+        elements = self._data.keys()
+        if sorted:
+            elements.sort()
+        return '%s(%r)' % (self.__class__.__name__, elements)
+
+    def __iter__(self):
+        """Return an iterator over the elements or a set.
+
+        This is the keys iterator for the underlying dict.
+        """
+        return self._data.iterkeys()
+
+    # Three-way comparison is not supported.  However, because __eq__ is
+    # tried before __cmp__, if Set x == Set y, x.__eq__(y) returns True and
+    # then cmp(x, y) returns 0 (Python doesn't actually call __cmp__ in this
+    # case).
+
+    def __cmp__(self, other):
+        raise TypeError, "can't compare sets using cmp()"
+
+    # Equality comparisons using the underlying dicts.  Mixed-type comparisons
+    # are allowed here, where Set == z for non-Set z always returns False,
+    # and Set != z always True.  This allows expressions like "x in y" to
+    # give the expected result when y is a sequence of mixed types, not
+    # raising a pointless TypeError just because y contains a Set, or x is
+    # a Set and y contain's a non-set ("in" invokes only __eq__).
+    # Subtle:  it would be nicer if __eq__ and __ne__ could return
+    # NotImplemented instead of True or False.  Then the other comparand
+    # would get a chance to determine the result, and if the other comparand
+    # also returned NotImplemented then it would fall back to object address
+    # comparison (which would always return False for __eq__ and always
+    # True for __ne__).  However, that doesn't work, because this type
+    # *also* implements __cmp__:  if, e.g., __eq__ returns NotImplemented,
+    # Python tries __cmp__ next, and the __cmp__ here then raises TypeError.
+
+    def __eq__(self, other):
+        if isinstance(other, BaseSet):
+            return self._data == other._data
+        else:
+            return False
+
+    def __ne__(self, other):
+        if isinstance(other, BaseSet):
+            return self._data != other._data
+        else:
+            return True
+
+    # Copying operations
+
+    def copy(self):
+        """Return a shallow copy of a set."""
+        result = self.__class__()
+        result._data.update(self._data)
+        return result
+
+    __copy__ = copy # For the copy module
+
+    def __deepcopy__(self, memo):
+        """Return a deep copy of a set; used by copy module."""
+        # This pre-creates the result and inserts it in the memo
+        # early, in case the deep copy recurses into another reference
+        # to this same set.  A set can't be an element of itself, but
+        # it can certainly contain an object that has a reference to
+        # itself.
+        from copy import deepcopy
+        result = self.__class__()
+        memo[id(self)] = result
+        data = result._data
+        value = True
+        for elt in self:
+            data[deepcopy(elt, memo)] = value
+        return result
+
+    # Standard set operations: union, intersection, both differences.
+    # Each has an operator version (e.g. __or__, invoked with |) and a
+    # method version (e.g. union).
+    # Subtle:  Each pair requires distinct code so that the outcome is
+    # correct when the type of other isn't suitable.  For example, if
+    # we did "union = __or__" instead, then Set().union(3) would return
+    # NotImplemented instead of raising TypeError (albeit that *why* it
+    # raises TypeError as-is is also a bit subtle).
+
+    def __or__(self, other):
+        """Return the union of two sets as a new set.
+
+        (I.e. all elements that are in either set.)
+        """
+        if not isinstance(other, BaseSet):
+            return NotImplemented
+        return self.union(other)
+
+    def union(self, other):
+        """Return the union of two sets as a new set.
+
+        (I.e. all elements that are in either set.)
+        """
+        result = self.__class__(self)
+        result._update(other)
+        return result
+
+    def __and__(self, other):
+        """Return the intersection of two sets as a new set.
+
+        (I.e. all elements that are in both sets.)
+        """
+        if not isinstance(other, BaseSet):
+            return NotImplemented
+        return self.intersection(other)
+
+    def intersection(self, other):
+        """Return the intersection of two sets as a new set.
+
+        (I.e. all elements that are in both sets.)
+        """
+        if not isinstance(other, BaseSet):
+            other = Set(other)
+        if len(self) <= len(other):
+            little, big = self, other
+        else:
+            little, big = other, self
+        common = ifilter(big._data.has_key, little)
+        return self.__class__(common)
+
+    def __xor__(self, other):
+        """Return the symmetric difference of two sets as a new set.
+
+        (I.e. all elements that are in exactly one of the sets.)
+        """
+        if not isinstance(other, BaseSet):
+            return NotImplemented
+        return self.symmetric_difference(other)
+
+    def symmetric_difference(self, other):
+        """Return the symmetric difference of two sets as a new set.
+
+        (I.e. all elements that are in exactly one of the sets.)
+        """
+        result = self.__class__()
+        data = result._data
+        value = True
+        selfdata = self._data
+        try:
+            otherdata = other._data
+        except AttributeError:
+            otherdata = Set(other)._data
+        for elt in ifilterfalse(otherdata.has_key, selfdata):
+            data[elt] = value
+        for elt in ifilterfalse(selfdata.has_key, otherdata):
+            data[elt] = value
+        return result
+
+    def  __sub__(self, other):
+        """Return the difference of two sets as a new Set.
+
+        (I.e. all elements that are in this set and not in the other.)
+        """
+        if not isinstance(other, BaseSet):
+            return NotImplemented
+        return self.difference(other)
+
+    def difference(self, other):
+        """Return the difference of two sets as a new Set.
+
+        (I.e. all elements that are in this set and not in the other.)
+        """
+        result = self.__class__()
+        data = result._data
+        try:
+            otherdata = other._data
+        except AttributeError:
+            otherdata = Set(other)._data
+        value = True
+        for elt in ifilterfalse(otherdata.has_key, self):
+            data[elt] = value
+        return result
+
+    # Membership test
+
+    def __contains__(self, element):
+        """Report whether an element is a member of a set.
+
+        (Called in response to the expression `element in self'.)
+        """
+        try:
+            return element in self._data
+        except TypeError:
+            transform = getattr(element, "__as_temporarily_immutable__", None)
+            if transform is None:
+                raise # re-raise the TypeError exception we caught
+            return transform() in self._data
+
+    # Subset and superset test
+
+    def issubset(self, other):
+        """Report whether another set contains this set."""
+        self._binary_sanity_check(other)
+        if len(self) > len(other):  # Fast check for obvious cases
+            return False
+        for elt in ifilterfalse(other._data.has_key, self):
+            return False
+        return True
+
+    def issuperset(self, other):
+        """Report whether this set contains another set."""
+        self._binary_sanity_check(other)
+        if len(self) < len(other):  # Fast check for obvious cases
+            return False
+        for elt in ifilterfalse(self._data.has_key, other):
+            return False
+        return True
+
+    # Inequality comparisons using the is-subset relation.
+    __le__ = issubset
+    __ge__ = issuperset
+
+    def __lt__(self, other):
+        self._binary_sanity_check(other)
+        return len(self) < len(other) and self.issubset(other)
+
+    def __gt__(self, other):
+        self._binary_sanity_check(other)
+        return len(self) > len(other) and self.issuperset(other)
+
+    # Assorted helpers
+
+    def _binary_sanity_check(self, other):
+        # Check that the other argument to a binary operation is also
+        # a set, raising a TypeError otherwise.
+        if not isinstance(other, BaseSet):
+            raise TypeError, "Binary operation only permitted between sets"
+
+    def _compute_hash(self):
+        # Calculate hash code for a set by xor'ing the hash codes of
+        # the elements.  This ensures that the hash code does not depend
+        # on the order in which elements are added to the set.  This is
+        # not called __hash__ because a BaseSet should not be hashable;
+        # only an ImmutableSet is hashable.
+        result = 0
+        for elt in self:
+            result ^= hash(elt)
+        return result
+
+    def _update(self, iterable):
+        # The main loop for update() and the subclass __init__() methods.
+        data = self._data
+
+        # Use the fast update() method when a dictionary is available.
+        if isinstance(iterable, BaseSet):
+            data.update(iterable._data)
+            return
+
+        value = True
+
+        if type(iterable) in (list, tuple, xrange):
+            # Optimized: we know that __iter__() and next() can't
+            # raise TypeError, so we can move 'try:' out of the loop.
+            it = iter(iterable)
+            while True:
+                try:
+                    for element in it:
+                        data[element] = value
+                    return
+                except TypeError:
+                    transform = getattr(element, "__as_immutable__", None)
+                    if transform is None:
+                        raise # re-raise the TypeError exception we caught
+                    data[transform()] = value
+        else:
+            # Safe: only catch TypeError where intended
+            for element in iterable:
+                try:
+                    data[element] = value
+                except TypeError:
+                    transform = getattr(element, "__as_immutable__", None)
+                    if transform is None:
+                        raise # re-raise the TypeError exception we caught
+                    data[transform()] = value
+
+
+class ImmutableSet(BaseSet):
+    """Immutable set class."""
+
+    __slots__ = ['_hashcode']
+
+    # BaseSet + hashing
+
+    def __init__(self, iterable=None):
+        """Construct an immutable set from an optional iterable."""
+        self._hashcode = None
+        self._data = {}
+        if iterable is not None:
+            self._update(iterable)
+
+    def __hash__(self):
+        if self._hashcode is None:
+            self._hashcode = self._compute_hash()
+        return self._hashcode
+
+    def __getstate__(self):
+        return self._data, self._hashcode
+
+    def __setstate__(self, state):
+        self._data, self._hashcode = state
+
+class Set(BaseSet):
+    """ Mutable set class."""
+
+    __slots__ = []
+
+    # BaseSet + operations requiring mutability; no hashing
+
+    def __init__(self, iterable=None):
+        """Construct a set from an optional iterable."""
+        self._data = {}
+        if iterable is not None:
+            self._update(iterable)
+
+    def __getstate__(self):
+        # getstate's results are ignored if it is not
+        return self._data,
+
+    def __setstate__(self, data):
+        self._data, = data
+
+    def __hash__(self):
+        """A Set cannot be hashed."""
+        # We inherit object.__hash__, so we must deny this explicitly
+        raise TypeError, "Can't hash a Set, only an ImmutableSet."
+
+    # In-place union, intersection, differences.
+    # Subtle:  The xyz_update() functions deliberately return None,
+    # as do all mutating operations on built-in container types.
+    # The __xyz__ spellings have to return self, though.
+
+    def __ior__(self, other):
+        """Update a set with the union of itself and another."""
+        self._binary_sanity_check(other)
+        self._data.update(other._data)
+        return self
+
+    def union_update(self, other):
+        """Update a set with the union of itself and another."""
+        self._update(other)
+
+    def __iand__(self, other):
+        """Update a set with the intersection of itself and another."""
+        self._binary_sanity_check(other)
+        self._data = (self & other)._data
+        return self
+
+    def intersection_update(self, other):
+        """Update a set with the intersection of itself and another."""
+        if isinstance(other, BaseSet):
+            self &= other
+        else:
+            self._data = (self.intersection(other))._data
+
+    def __ixor__(self, other):
+        """Update a set with the symmetric difference of itself and another."""
+        self._binary_sanity_check(other)
+        self.symmetric_difference_update(other)
+        return self
+
+    def symmetric_difference_update(self, other):
+        """Update a set with the symmetric difference of itself and another."""
+        data = self._data
+        value = True
+        if not isinstance(other, BaseSet):
+            other = Set(other)
+        if self is other:
+            self.clear()
+        for elt in other:
+            if elt in data:
+                del data[elt]
+            else:
+                data[elt] = value
+
+    def __isub__(self, other):
+        """Remove all elements of another set from this set."""
+        self._binary_sanity_check(other)
+        self.difference_update(other)
+        return self
+
+    def difference_update(self, other):
+        """Remove all elements of another set from this set."""
+        data = self._data
+        if not isinstance(other, BaseSet):
+            other = Set(other)
+        if self is other:
+            self.clear()
+        for elt in ifilter(data.has_key, other):
+            del data[elt]
+
+    # Python dict-like mass mutations: update, clear
+
+    def update(self, iterable):
+        """Add all values from an iterable (such as a list or file)."""
+        self._update(iterable)
+
+    def clear(self):
+        """Remove all elements from this set."""
+        self._data.clear()
+
+    # Single-element mutations: add, remove, discard
+
+    def add(self, element):
+        """Add an element to a set.
+
+        This has no effect if the element is already present.
+        """
+        try:
+            self._data[element] = True
+        except TypeError:
+            transform = getattr(element, "__as_immutable__", None)
+            if transform is None:
+                raise # re-raise the TypeError exception we caught
+            self._data[transform()] = True
+
+    def remove(self, element):
+        """Remove an element from a set; it must be a member.
+
+        If the element is not a member, raise a KeyError.
+        """
+        try:
+            del self._data[element]
+        except TypeError:
+            transform = getattr(element, "__as_temporarily_immutable__", None)
+            if transform is None:
+                raise # re-raise the TypeError exception we caught
+            del self._data[transform()]
+
+    def discard(self, element):
+        """Remove an element from a set if it is a member.
+
+        If the element is not a member, do nothing.
+        """
+        try:
+            self.remove(element)
+        except KeyError:
+            pass
+
+    def pop(self):
+        """Remove and return an arbitrary set element."""
+        return self._data.popitem()[0]
+
+    def __as_immutable__(self):
+        # Return a copy of self as an immutable set
+        return ImmutableSet(self)
+
+    def __as_temporarily_immutable__(self):
+        # Return self wrapped in a temporarily immutable set
+        return _TemporarilyImmutableSet(self)
+
+
+class _TemporarilyImmutableSet(BaseSet):
+    # Wrap a mutable set as if it was temporarily immutable.
+    # This only supplies hashing and equality comparisons.
+
+    def __init__(self, set):
+        self._set = set
+        self._data = set._data  # Needed by ImmutableSet.__eq__()
+
+    def __hash__(self):
+        return self._set._compute_hash()
+
+# Local Variables:
+# tab-width:4
+# indent-tabs-mode:nil
+# End:
+# vim: set expandtab tabstop=4 shiftwidth=4: