code: func (d *Driver) isParent(id, parent string) bool {
  	lowers, err := d.getLowerDirs(id)
  	if err != nil {
  		return false
  	}
  	if parent == "" && len(lowers) > 0 {
  		return false
  	}
  
  	parentDir := d.dir(parent)
  	var ld string
  	if len(lowers) > 0 {
  		ld = filepath.Dir(lowers[0])
  	}
  	if ld == "" && parent == "" {
  		return true
  	}
  	return ld == parentDir
  }
docs: // isParent determines whether the given parent is the direct parent of the
  // given layer id