code: func (r *Router) NotFound(handlers ...Handler) {
  	handlers = validateAndWrapHandlers(handlers)
  	r.notFound = func(rw http.ResponseWriter, req *http.Request) {
  		c := r.m.createContext(rw, req)
  		c.handlers = make([]Handler, 0, len(r.m.handlers)+len(handlers))
  		c.handlers = append(c.handlers, r.m.handlers...)
  		c.handlers = append(c.handlers, handlers...)
  		c.run()
  	}
  }
docs: // NotFound configurates http.HandlerFunc which is called when no matching route is
  // found. If it is not set, http.NotFound is used.
  // Be sure to set 404 response code in your handler.