code: func (mux *Mux) Close() (err error) {
  	mux.once.Do(func() {
  		// Close underlying listener.
  		if mux.ln != nil {
  			err = mux.ln.Close()
  		}
  
  		// Wait for open connections to close and then close handlers.
  		mux.wg.Wait()
  		for _, h := range mux.handlers {
  			h.Close()
  		}
  	})
  	return
  }
docs: // Close closes the underlying listener.