code: def _killBackref( receiver, senderkey ):
      
      receiverkey = id(receiver)
      set = sendersBack.get( receiverkey, () )
      while senderkey in set:
          try:
              set.remove( senderkey )
          except:
              break
      if not set:
          try:
              del sendersBack[ receiverkey ]
          except KeyError:
              pass
      return True
docs: Do the actual removal of back reference from receiver to senderkey