code: func (d *Driver) resolveID(id string) (string, error) {
  	content, err := ioutil.ReadFile(filepath.Join(d.dir(id), "layerID"))
  	if os.IsNotExist(err) {
  		return id, nil
  	} else if err != nil {
  		return "", err
  	}
  	return string(content), nil
  }
docs: // resolveID computes the layerID information based on the given id.