code: func (r *Router) NotFound(mc MiddlewareChain, h Handler) {
  	handle := r.adapt(mc, h, "")
  	r.hrouter.NotFound = http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
  		handle(rw, req, nil)
  	})
  }
docs: // NotFound sets the handler to be called when no matching route is found.