code: func (id Identity) Validate() error {
  	chunks := strings.SplitN(string(id), ":", 2)
  	if len(chunks) != 2 {
  		return fmt.Errorf("auth: bad identity string %q", id)
  	}
  	re := knownKinds[Kind(chunks[0])]
  	if re == nil {
  		return fmt.Errorf("auth: bad identity kind %q", chunks[0])
  	}
  	if !re.MatchString(chunks[1]) {
  		return fmt.Errorf("auth: bad value %q for identity kind %q", chunks[1], chunks[0])
  	}
  	return nil
  }
docs: // Validate checks that the identity string is well-formed.