code: func (g *WriterGroup) Add(key string, w io.WriteCloser) {
  	g.mu.Lock()
  	defer g.mu.Unlock()
  	if g.closed {
  		w.Close()
  		return
  	}
  	g.writers[key] = w
  }
docs: // Add adds a writer into the group. The writer will be closed
  // if the writer group is closed.