code: func (b Box) WalkPrefix(prefix string, wf WalkFunc) error {
  	opre := osPath(prefix)
  	return b.Walk(func(path string, f File) error {
  		if strings.HasPrefix(osPath(path), opre) {
  			if err := wf(path, f); err != nil {
  				return err
  			}
  		}
  		return nil
  	})
  }
docs: // WalkPrefix will call box.Walk and call the WalkFunc when it finds paths that have a matching prefix