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diff --git a/src/engine/SCons/compat/_scons_sets15.py b/src/engine/SCons/compat/_scons_sets15.py
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+#
+# A Set class that works all the way back to Python 1.5.  From:
+#
+#       Python Cookbook:  Yet another Set class for Python
+#       http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/106469
+#       Goncalo Rodriques
+#
+#       This is a pure Pythonic implementation of a set class.  The syntax
+#       and methods implemented are, for the most part, borrowed from
+#       PEP 218 by Greg Wilson.
+#
+# Note that this class violates the formal definition of a set() by adding
+# a __getitem__() method so we can iterate over a set's elements under
+# Python 1.5 and 2.1, which don't support __iter__() and iterator types.
+#
+
+import string
+
+class Set:
+    """The set class. It can contain mutable objects."""
+
+    def __init__(self, seq = None):
+        """The constructor. It can take any object giving an iterator as an optional
+        argument to populate the new set."""
+        self.elems = []
+        if seq:
+            for elem in seq:
+                if elem not in self.elems:
+                    hash(elem)
+                    self.elems.append(elem)
+
+    def __str__(self):
+        return "set([%s])" % string.join(map(str, self.elems), ", ")
+
+
+    def copy(self):
+        """Shallow copy of a set object."""
+        return Set(self.elems)
+
+    def __contains__(self, elem):
+        return elem in self.elems
+
+    def __len__(self):
+        return len(self.elems)
+
+    def __getitem__(self, index):
+        # Added so that Python 1.5 can iterate over the elements.
+        # The cookbook recipe's author didn't like this because there
+        # really isn't any order in a set object, but this is necessary
+        # to make the class work well enough for our purposes.
+        return self.elems[index]
+
+    def items(self):
+        """Returns a list of the elements in the set."""
+        return self.elems
+
+    def add(self, elem):
+        """Add one element to the set."""
+        if elem not in self.elems:
+            hash(elem)
+            self.elems.append(elem)
+
+    def remove(self, elem):
+        """Remove an element from the set. Return an error if elem is not in the set."""
+        try:
+            self.elems.remove(elem)
+        except ValueError:
+            raise LookupError, "Object %s is not a member of the set." % str(elem)
+
+    def discard(self, elem):
+        """Remove an element from the set. Do nothing if elem is not in the set."""
+        try:
+            self.elems.remove(elem)
+        except ValueError:
+            pass
+
+    def sort(self, func=cmp):
+        self.elems.sort(func)
+
+    #Define an iterator for a set.
+    def __iter__(self):
+        return iter(self.elems)
+
+    #The basic binary operations with sets.
+    def __or__(self, other):
+        """Union of two sets."""
+        ret = self.copy()
+        for elem in other.elems:
+            if elem not in ret:
+                ret.elems.append(elem)
+        return ret
+
+    def __sub__(self, other):
+        """Difference of two sets."""
+        ret = self.copy()
+        for elem in other.elems:
+            ret.discard(elem)
+        return ret
+
+    def __and__(self, other):
+        """Intersection of two sets."""
+        ret = Set()
+        for elem in self.elems:
+            if elem in other.elems:
+                ret.elems.append(elem)
+        return ret
+
+    def __add__(self, other):
+        """Symmetric difference of two sets."""
+        ret = Set()
+        temp = other.copy()
+        for elem in self.elems:
+            if elem in temp.elems:
+                temp.elems.remove(elem)
+            else:
+                ret.elems.append(elem)
+        #Add remaining elements.
+        for elem in temp.elems:
+                ret.elems.append(elem)
+        return ret
+
+    def __mul__(self, other):
+        """Cartesian product of two sets."""
+        ret = Set()
+        for elemself in self.elems:
+            x = map(lambda other, s=elemself: (s, other), other.elems)
+            ret.elems.extend(x)
+        return ret
+
+    #Some of the binary comparisons.
+    def __lt__(self, other):
+        """Returns 1 if the lhs set is contained but not equal to the rhs set."""
+        if len(self.elems) < len(other.elems):
+            temp = other.copy()
+            for elem in self.elems:
+                if elem in temp.elems:
+                    temp.remove(elem)
+                else:
+                    return 0
+            return len(temp.elems) == 0
+        else:
+            return 0
+
+    def __le__(self, other):
+        """Returns 1 if the lhs set is contained in the rhs set."""
+        if len(self.elems) <= len(other.elems):
+            ret = 1
+            for elem in self.elems:
+                if elem not in other.elems:
+                    ret = 0
+                    break
+            return ret
+        else:
+            return 0
+
+    def __eq__(self, other):
+        """Returns 1 if the sets are equal."""
+        if len(self.elems) != len(other.elems):
+            return 0
+        else:
+            return len(self - other) == 0
+
+    def __cmp__(self, other):
+        """Returns 1 if the sets are equal."""
+        if self.__lt__(other):
+            return -1
+        elif other.__lt__(self):
+            return 1
+        else:
+            return 0
+
+# Local Variables:
+# tab-width:4
+# indent-tabs-mode:nil
+# End:
+# vim: set expandtab tabstop=4 shiftwidth=4: