code: func WalkFiles(fs http.FileSystem, root string, walkFn WalkFilesFunc) error {
  	file, info, err := openStat(fs, root)
  	if err != nil {
  		return walkFn(root, nil, nil, err)
  	}
  	return walkFiles(fs, root, info, file, walkFn)
  }
docs: // WalkFiles walks the filesystem rooted at root, calling walkFn for each file or
  // directory in the filesystem, including root. In addition to FileInfo, it passes an
  // ReadSeeker to walkFn for each file it visits.