Index: trellis.py
===================================================================
--- trellis.py	(revision 2599)
+++ trellis.py	(working copy)
@@ -207,9 +219,17 @@
         if self.next_subject is None and (
             self._reset is _sentinel or self._value==self._reset
         ):
+            link = self.next_listener
+            while link is not None:
+                nxt = link.next_listener
+                listener = link()
+                if isinstance(listener, ReadOnlyCell) and listener.next_subject is None:
+                    listener._check_const()
+                on_undo(stm.Link, self, listener)
+                link.unlink()
+                link = nxt
             change_attr(self, '_set_by', _sentinel)
             change_attr(self, 'rule', None)
-            change_attr(self, 'next_listener', None)
             change_attr(self, '__class__', self._const_class())

     def _const_class(self):