code: func (c *Client) notifySpentInternal(outpoints []btcjson.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()
  	}
  
  	cmd := btcjson.NewNotifySpentCmd(outpoints)
  	return c.sendCmd(cmd)
  }
docs: // notifySpentInternal is the same as notifySpentAsync except it accepts
  // the converted outpoints as a parameter so the client can more efficiently
  // recreate the previous notification state on reconnect.