code: func NotFound(w http.ResponseWriter, r *http.Request) {
  	handlerMu.RLock()
  	f, ok := handlerMap[http.StatusNotFound]
  	handlerMu.RUnlock()
  	if ok {
  		f.ServeHTTP(w, r)
  	} else {
  		defaultNotFound(w, r)
  	}
  }
docs: // NotFound returns a 404 Not Found error to the client.