code: def _addToSegmentUpdates(self, c, i, segUpdate):
      
      # Sometimes we might be passed an empty update
      if segUpdate is None or len(segUpdate.activeSynapses) == 0:
        return
  
      key = (c, i) # key = (column index, cell index in column)
  
      # TODO: scan list of updates for that cell and consolidate?
      # But watch out for dates!
      if self.segmentUpdates.has_key(key):
        self.segmentUpdates[key] += [(self.lrnIterationIdx, segUpdate)]
      else:
        self.segmentUpdates[key] = [(self.lrnIterationIdx, segUpdate)]
docs: Store a dated potential segment update. The "date" (iteration index) is used
      later to determine whether the update is too old and should be forgotten.
      This is controlled by parameter ``segUpdateValidDuration``.
  
      :param c: TODO: document
      :param i: TODO: document
      :param segUpdate: TODO: document