code: func (gv GroupVersion) MarshalJSON() ([]byte, error) {
  	s := gv.String()
  	if strings.Count(s, "/") > 1 {
  		return []byte{}, fmt.Errorf("illegal GroupVersion %v: contains more than one /", s)
  	}
  	return json.Marshal(s)
  }
docs: // MarshalJSON implements the json.Marshaller interface.