code: func (b Box) List() []string {
  	var keys []string
  
  	if b.data == nil || len(b.data) == 0 {
  		b.Walk(func(path string, info File) error {
  			finfo, _ := info.FileInfo()
  			if !finfo.IsDir() {
  				keys = append(keys, finfo.Name())
  			}
  			return nil
  		})
  	} else {
  		for k := range b.data {
  			keys = append(keys, k)
  		}
  	}
  	return keys
  }
docs: // List shows "What's in the box?"