code: func backup(filename string) error {
  	if DO_BACKUP {
  		if _, ok := _FILES_BACKUPED[filename]; !ok {
  			if err := file.Backup(filename); err != nil {
  				return err
  			}
  			_FILES_BACKUPED[filename] = struct{}{}
  		}
  	}
  	return nil
  }
docs: // backup does a backup of a file.