code: def _cleanup(self, ref):
          'cleanup after a transitor weakref fires'
          self.transitor_states[self._weakref_holder[ref]] -= 1
          del self._weakref_holder[reff _cleanup(self, ref):
          'cleanup after a transitor weakref fires'
          self.transitor_states[self._weakref_holder[ref]] -= 1
          del self._weakref_holder[ref]
docs: cleanup after a transitor weakref fires