code: func (s *FileStorage) MostRecentUserEmail() string {
  	userDirs, err := ioutil.ReadDir(s.users())
  	if err != nil {
  		return ""
  	}
  	var mostRecent os.FileInfo
  	for _, dir := range userDirs {
  		if !dir.IsDir() {
  			continue
  		}
  		if mostRecent == nil || dir.ModTime().After(mostRecent.ModTime()) {
  			mostRecent = dir
  		}
  	}
  	if mostRecent != nil {
  		return mostRecent.Name()
  	}
  	return ""
  }
docs: // MostRecentUserEmail implements Storage.MostRecentUserEmail by finding the
  // most recently written sub directory in the users' directory. It is named
  // after the email address. This corresponds to the most recent call to
  // StoreUser.