code: func messageTargetAsStringsArray(m HttpMessage) ([]string, error) {
  	if m.RegistrationIds != nil {
  		return m.RegistrationIds, nil
  	} else if m.To != "" {
  		target := []string{m.To}
  		return target, nil
  	}
  	target := []string{}
  	return target, fmt.Errorf("can't find any valid target field in message.")
  }
docs: // Transform the recipient in an array of strings if needed.