code: func (m *mux) HandleNotFound(handle MuxHandler) {
  	nfh := func(rw http.ResponseWriter, req *http.Request) {
  		handle(rw, req, nil)
  	}
  	m.router.NotFoundHandler = nfh
  }
docs: // HandleNotFound sets the MuxHandler invoked for requests that don't match any
  // handler registered with Handle.