code: func (mw *MirrorWriter) AddWriter(w io.WriteCloser) {
  	wa := &writerAdd{
  		w:    w,
  		done: make(chan struct{}),
  	}
  	mw.writerAdd <- wa
  	<-wa.done
  }
docs: // AddWriter attaches a new WriteCloser to this MirrorWriter.
  // The new writer will start getting any bytes written to the mirror.