code: func (ws *AtomicWriteSet) FileWriter(name string, flag int, perm os.FileMode) (io.WriteCloser, error) {
  	f, err := os.OpenFile(filepath.Join(ws.root, name), flag, perm)
  	if err != nil {
  		return nil, err
  	}
  	return syncFileCloser{f}, nil
  }
docs: // FileWriter opens a file writer inside the set. The file
  // should be synced and closed before calling commit.