code: func (id Identity) Email() string {
  	chunks := strings.SplitN(string(id), ":", 2)
  	if len(chunks) != 2 {
  		return ""
  	}
  	if Kind(chunks[0]) == User {
  		return chunks[1]
  	}
  	return ""
  }
docs: // Email returns user's email for identity with kind User or empty string for
  // all other identity kinds. If identity string is undefined returns "".