code: func (f *File) Lookup(name string) (*File, error) {
  	var bino uint32
  	bname := []byte(name)
  	err := f.walkDir(func(name []byte, ino uint32) bool {
  		if bytes.Equal(bname, name) {
  			bino = ino
  			return false
  		}
  		return true
  	})
  	if err != nil {
  		return nil, err
  	}
  	if bino == 0 {
  		return nil, fmt.Errorf("file not found")
  	}
  	return f.fs.File(bino)
  }
docs: // Lookup looks up the name in the directory f, returning the corresponding child file.