Index: stm.py
===================================================================
--- stm.py	(revision 2595)
+++ stm.py	(working copy)
@@ -1,6 +1,6 @@
 """Software Transactional Memory and Observers"""

-import weakref, sys, heapq, UserList, UserDict, sets
+import weakref, sys, heapq, UserList, UserDict
 from peak.util.extremes import Max
 from peak.util import decorators
 try:
Index: trellis.py
===================================================================
--- trellis.py	(revision 2595)
+++ trellis.py	(working copy)
@@ -1,7 +1,7 @@
 from thread import get_ident
 from weakref import ref
 from peak.util import addons, decorators
-import sys, UserDict, UserList, sets, stm, types, new, weakref, copy
+import sys, UserDict, UserList, stm, types, new, weakref, copy
 from peak.util.extremes import Max
 from peak.util.symbols import Symbol, NOT_GIVEN

@@ -42,12 +42,13 @@
 try:
     set = set
 except NameError:
+    import sets
     set = sets.Set
     frozenset = sets.ImmutableSet
     set_like = sets.BaseSet
     dictlike = dict, sets.BaseSet
 else:
-    set_like = set, frozenset, sets.BaseSet
+    set_like = set, frozenset
     dictlike = (dict,) + set_like


@@ -506,7 +508,7 @@
                 return ReadOnlyCell(rule, None, discrete)
             elif rule is None:
                 return Value(v, discrete)
-        return ReadOnlyCell.__new__(cls, rule, value, discrete)
+        return ReadOnlyCell.__new__(cls)#, rule, value, discrete)

     def _check_const(self):
         pass    # we can never become Constant
@@ -529,8 +531,9 @@
         super(Cell, self).set_value(value)
         if self._needs_init:
             schedule(self)
+        else:
+            cancel(self)

-
     value = property(get_value, set_value)

     def dirty(self):
@@ -748,6 +751,8 @@
         except KeyError:
             name = self.__name__
             cell = cells.setdefault(name, self.make_cell(typ, ob, name))
+            if ctrl.active:
+                on_undo(cells.pop, name)
         return cell.value

     def __repr__(self):
@@ -1240,6 +1245,9 @@
     strategy is used in each recalcultion that changes the list.  If what you
     really want is e.g. a sorted read-only view on a set, don't use this.
     """
+    if hasattr(UserList.UserList, '__metaclass__'):
+        class __metaclass__(Component.__metaclass__, UserList.UserList.__metaclass__):
+            pass

     updated = todo(lambda self: self.data[:])
     future  = updated.future
@@ -1351,7 +1359,7 @@



-class Set(sets.Set, Component):
+class Set(set, Component):
     """Mutable set that recalculates observers when it's changed

     The ``added`` and ``removed`` attributes can be watched for changes, but