code: func Walk(ctx context.Context, root string, fs FileSystem) *Walker {
  	info, err := fs.Lstat(ctx, root)
  	return &Walker{
  		fs:    fs,
  		ctx:   ctx,
  		stack: []item{{root, info, err}},
  	}
  }
docs: // Walk returns a new Walker rooted at root on the FileSystem fs.