code: func readDirNames(fs http.FileSystem, dirname string) ([]string, error) {
  	fis, err := ReadDir(fs, dirname)
  	if err != nil {
  		return nil, err
  	}
  	names := make([]string, len(fis))
  	for i := range fis {
  		names[i] = fis[i].Name()
  	}
  	sort.Strings(names)
  	return names, nil
  }
docs: // readDirNames reads the directory named by dirname and returns
  // a sorted list of directory entries.