code: func handleNotifySpent(wsc *wsClient, icmd interface{}) (interface{}, error) {
  	cmd, ok := icmd.(*btcjson.NotifySpentCmd)
  	if !ok {
  		return nil, btcjson.ErrRPCInternal
  	}
  
  	outpoints, err := deserializeOutpoints(cmd.OutPoints)
  	if err != nil {
  		return nil, err
  	}
  
  	wsc.server.ntfnMgr.RegisterSpentRequests(wsc, outpoints)
  	return nil, nil
  }
docs: // handleNotifySpent implements the notifyspent command extension for
  // websocket connections.