diff --git a/sylk/thirdparty/__init__.py b/sylk/thirdparty/__init__.py
new file mode 100644
index 0000000..abdbbc0
--- /dev/null
+++ b/sylk/thirdparty/__init__.py
@@ -0,0 +1,3 @@
+# Copyright (C) 2011 AG Projects. See LICENSE for details.
+#
+
diff --git a/sylk/thirdparty/weakrefset.py b/sylk/thirdparty/weakrefset.py
new file mode 100644
index 0000000..072c453
--- /dev/null
+++ b/sylk/thirdparty/weakrefset.py
@@ -0,0 +1,213 @@
+from __future__ import with_statement
+
+from _weakref import ref
+
+__all__ = ['WeakSet']
+
+__version__ = '1.0.0'
+
+
+
+class _IterationGuard(object):
+    # This context manager registers itself in the current iterators of the
+    # weak container, such as to delay all removals until the context manager
+    # exits.
+    # This technique should be relatively thread-safe (since sets are).
+
+    def __init__(self, weakcontainer):
+        # Don't create cycles
+        self.weakcontainer = ref(weakcontainer)
+
+    def __enter__(self):
+        w = self.weakcontainer()
+        if w is not None:
+            w._iterating.add(self)
+        return self
+
+    def __exit__(self, e, t, b):
+        w = self.weakcontainer()
+        if w is not None:
+            s = w._iterating
+            s.remove(self)
+            if not s:
+                w._commit_removals()
+
+
+class WeakSet(object):
+    def __init__(self, data=None):
+        self.data = set()
+        def _remove(item, selfref=ref(self)):
+            self = selfref()
+            if self is not None:
+                if self._iterating:
+                    self._pending_removals.append(item)
+                else:
+                    self.data.discard(item)
+        self._remove = _remove
+        # A list of keys to be removed
+        self._pending_removals = []
+        self._iterating = set()
+        if data is not None:
+            self.update(data)
+
+    def _commit_removals(self):
+        l = self._pending_removals
+        discard = self.data.discard
+        while l:
+            discard(l.pop())
+
+    def __iter__(self):
+        with _IterationGuard(self):
+            for itemref in self.data:
+                item = itemref()
+                if item is not None:
+                    yield item
+
+    def __len__(self):
+        return sum(x() is not None for x in self.data)
+
+    def __contains__(self, item):
+        return ref(item) in self.data
+
+    def __reduce__(self):
+        return (self.__class__, (list(self),),
+                getattr(self, '__dict__', None))
+
+    __hash__ = None
+
+    def add(self, item):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.add(ref(item, self._remove))
+
+    def clear(self):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.clear()
+
+    def copy(self):
+        return self.__class__(self)
+
+    def pop(self):
+        if self._pending_removals:
+            self._commit_removals()
+        while True:
+            try:
+                itemref = self.data.pop()
+            except KeyError:
+                raise KeyError('pop from empty WeakSet')
+            item = itemref()
+            if item is not None:
+                return item
+
+    def remove(self, item):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.remove(ref(item))
+
+    def discard(self, item):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.discard(ref(item))
+
+    def update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        if isinstance(other, self.__class__):
+            self.data.update(other.data)
+        else:
+            for element in other:
+                self.add(element)
+
+    def __ior__(self, other):
+        self.update(other)
+        return self
+
+    # Helper functions for simple delegating methods.
+    def _apply(self, other, method):
+        if not isinstance(other, self.__class__):
+            other = self.__class__(other)
+        newdata = method(other.data)
+        newset = self.__class__()
+        newset.data = newdata
+        return newset
+
+    def difference(self, other):
+        return self._apply(other, self.data.difference)
+    __sub__ = difference
+
+    def difference_update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        if self is other:
+            self.data.clear()
+        else:
+            self.data.difference_update(ref(item) for item in other)
+    def __isub__(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        if self is other:
+            self.data.clear()
+        else:
+            self.data.difference_update(ref(item) for item in other)
+        return self
+
+    def intersection(self, other):
+        return self._apply(other, self.data.intersection)
+    __and__ = intersection
+
+    def intersection_update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.intersection_update(ref(item) for item in other)
+    def __iand__(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.intersection_update(ref(item) for item in other)
+        return self
+
+    def issubset(self, other):
+        return self.data.issubset(ref(item) for item in other)
+    __lt__ = issubset
+
+    def __le__(self, other):
+        return self.data <= set(ref(item) for item in other)
+
+    def issuperset(self, other):
+        return self.data.issuperset(ref(item) for item in other)
+    __gt__ = issuperset
+
+    def __ge__(self, other):
+        return self.data >= set(ref(item) for item in other)
+
+    def __eq__(self, other):
+        if not isinstance(other, self.__class__):
+            return NotImplemented
+        return self.data == set(ref(item) for item in other)
+
+    def symmetric_difference(self, other):
+        return self._apply(other, self.data.symmetric_difference)
+    __xor__ = symmetric_difference
+
+    def symmetric_difference_update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        if self is other:
+            self.data.clear()
+        else:
+            self.data.symmetric_difference_update(ref(item) for item in other)
+    def __ixor__(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        if self is other:
+            self.data.clear()
+        else:
+            self.data.symmetric_difference_update(ref(item) for item in other)
+        return self
+
+    def union(self, other):
+        return self._apply(other, self.data.union)
+    __or__ = union
+
+    def isdisjoint(self, other):
+        return len(self.intersection(other)) == 0