code: func (c *Client) NotifySpentAsync(outpoints []*wire.OutPoint) FutureNotifySpentResult {
  	// Not supported in HTTP POST mode.
  	if c.config.HTTPPostMode {
  		return newFutureError(ErrWebsocketsRequired)
  	}
  
  	// Ignore the notification if the client is not interested in
  	// notifications.
  	if c.ntfnHandlers == nil {
  		return newNilFutureResult()
  	}
  
  	ops := make([]btcjson.OutPoint, 0, len(outpoints))
  	for _, outpoint := range outpoints {
  		ops = append(ops, newOutPointFromWire(outpoint))
  	}
  	cmd := btcjson.NewNotifySpentCmd(ops)
  	return c.sendCmd(cmd)
  }
docs: // NotifySpentAsync returns an instance of a type that can be used to get the
  // result of the RPC at some future time by invoking the Receive function on
  // the returned instance.
  //
  // See NotifySpent for the blocking version and more details.
  //
  // NOTE: This is a btcd extension and requires a websocket connection.
  //
  // NOTE: Deprecated. Use LoadTxFilterAsync instead.