code: func (sp *Suback) String() string {
  	var codes []string
  
  	for _, c := range sp.ReturnCodes {
  		codes = append(codes, fmt.Sprintf("%d", c))
  	}
  
  	return fmt.Sprintf("<Suback ID=%d ReturnCodes=[%s]>",
  		sp.ID, strings.Join(codes, ", "))
  }
docs: // String returns a string representation of the packet.