code: func (d *DiskStorer) Del(key string) error {
  	if !validKey(key) {
  		return errNoSession{}
  	}
  
  	filePath := path.Join(d.folderPath, key)
  
  	d.mut.Lock()
  	defer d.mut.Unlock()
  
  	_, err := os.Stat(filePath)
  	if os.IsNotExist(err) {
  		return nil
  	} else if err != nil {
  		return errors.Wrapf(err, "unable to stat session file: %s", filePath)
  	}
  
  	return os.Remove(filePath)
  }
docs: // Del the session pointed to by the session id key and remove it.