code: func (c *Client) DebugLevelAsync(levelSpec string) FutureDebugLevelResult {
  	cmd := btcjson.NewDebugLevelCmd(levelSpec)
  	return c.sendCmd(cmd)
  }
docs: // DebugLevelAsync 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 DebugLevel for the blocking version and more details.
  //
  // NOTE: This is a btcd extension.