code: def cleanUpdatesList(self, col, cellIdx, seg):
      
      for key, updateList in self.segmentUpdates.iteritems():
        c,i = key[0], key[1]
        if c == col and i == cellIdx:
          for update in updateList:
            if update[1].segment == seg:
              self.removeSegmentUpdate(update)
docs: Removes any update that would be for the given col, cellIdx, segIdx.
      NOTE: logically, we need to do this when we delete segments, so that if
      an update refers to a segment that was just deleted, we also remove
      that update from the update list. However, I haven't seen it trigger
      in any of the unit tests yet, so it might mean that it's not needed
      and that situation doesn't occur, by construction.
      todo: check if that situation occurs.