code: func (r *Router) NotFound(fn interface{}) *Router {
  	if r.parent != nil {
  		panic("You can only set a NotFoundHandler on the root router.")
  	}
  	vfn := reflect.ValueOf(fn)
  	validateNotFoundHandler(vfn, r.contextType)
  	r.notFoundHandler = vfn
  	return r
  }
docs: // NotFound sets the specified function as the not-found handler (when no route matches) and returns the router.
  // Note that only the root router can have a NotFound handler.