code: func (c *Client) NotifySpent(outpoints []*wire.OutPoint) error {
  	return c.NotifySpentAsync(outpoints).Receive()
  }
docs: // NotifySpent registers the client to receive notifications when the passed
  // transaction outputs are spent.  The notifications are delivered to the
  // notification handlers associated with the client.  Calling this function has
  // no effect if there are no notification handlers and will result in an error
  // if the client is configured to run in HTTP POST mode.
  //
  // The notifications delivered as a result of this call will be via
  // OnRedeemingTx.
  //
  // NOTE: This is a btcd extension and requires a websocket connection.
  //
  // NOTE: Deprecated. Use LoadTxFilter instead.