code: func (r *ServerResponse) Encode(w io.Writer) error {
  	if len(r.ACKs) > 1 {
  		return errors.New("multi_ack and multi_ack_detailed are not supported")
  	}
  
  	e := pktline.NewEncoder(w)
  	if len(r.ACKs) == 0 {
  		return e.Encodef("%s\n", nak)
  	}
  
  	return e.Encodef("%s %s\n", ack, r.ACKs[0].String())
  }
docs: // Encode encodes the ServerResponse into a writer.