code: func (f *Filesystem) add(name string, fi os.FileInfo) (err error) {
  	var file ReadableFile
  
  	if runtime.GOOS == "darwin" {
  		// When a file system is HFS+, its filepath is in NFD form.
  		name = norm.NFC.String(name)
  	}
  
  	file = f.SourceSpec.NewFileInfo(f.Base, name, false, fi)
  	f.files = append(f.files, file)
  
  	return err
  }
docs: // add populates a file in the Filesystem.files