code: func (p *pingDelegate) AckPayload() []byte {
  	var buf bytes.Buffer
  
  	// The first byte is the version number, forming a simple header.
  	version := []byte{PingVersion}
  	buf.Write(version)
  
  	// The rest of the message is the serialized coordinate.
  	enc := codec.NewEncoder(&buf, &codec.MsgpackHandle{})
  	if err := enc.Encode(p.serf.coordClient.GetCoordinate()); err != nil {
  		p.serf.logger.Printf("[ERR] serf: Failed to encode coordinate: %v\n", err)
  	}
  	return buf.Bytes()
  }
docs: // AckPayload is called to produce a payload to send back in response to a ping
  // request.