code: func Walk(fs http.FileSystem, root string, walkFn filepath.WalkFunc) error {
  	info, err := Stat(fs, root)
  	if err != nil {
  		return walkFn(root, nil, err)
  	}
  	return walk(fs, root, info, walkFn)
  }
docs: // Walk walks the filesystem rooted at root, calling walkFn for each file or
  // directory in the filesystem, including root. All errors that arise visiting files
  // and directories are filtered by walkFn. The files are walked in lexical
  // order.