code: func CreateNativeZeroLengthFiles(info *metainfo.Info, dir string) (err error) {
  	for _, fi := range info.UpvertedFiles() {
  		if fi.Length != 0 {
  			continue
  		}
  		name := filepath.Join(append([]string{dir, info.Name}, fi.Path...)...)
  		os.MkdirAll(filepath.Dir(name), 0777)
  		var f io.Closer
  		f, err = os.Create(name)
  		if err != nil {
  			break
  		}
  		f.Close()
  	}
  	return
  }
docs: // Creates natives files for any zero-length file entries in the info. This is
  // a helper for file-based storages, which don't address or write to zero-
  // length files because they have no corresponding pieces.