code: func (s *FileStorage) userRegFile(email string) string {
  	if email == "" {
  		email = emptyEmail
  	}
  	email = strings.ToLower(email)
  	fileName := emailUsername(email)
  	if fileName == "" {
  		fileName = "registration"
  	}
  	return filepath.Join(s.user(email), fileName+".json")
  }
docs: // userRegFile gets the path to the registration file for the user with the
  // given email address.