code: func (b *Box) List() []string {
  	var keys []string
  
  	b.Walk(func(path string, info File) error {
  		if info == nil {
  			return nil
  		}
  		finfo, _ := info.FileInfo()
  		if !finfo.IsDir() {
  			keys = append(keys, path)
  		}
  		return nil
  	})
  	sort.Strings(keys)
  	return keys
  }
docs: // List shows "What's in the box?"