code: func (mx *Mux) NotFound(handlerFn http.HandlerFunc) {
  	// Build NotFound handler chain
  	m := mx
  	hFn := handlerFn
  	if mx.inline && mx.parent != nil {
  		m = mx.parent
  		hFn = Chain(mx.middlewares...).HandlerFunc(hFn).ServeHTTP
  	}
  
  	// Update the notFoundHandler from this point forward
  	m.notFoundHandler = hFn
  	m.updateSubRoutes(func(subMux *Mux) {
  		if subMux.notFoundHandler == nil {
  			subMux.NotFound(hFn)
  		}
  	})
  }
docs: // NotFound sets a custom http.HandlerFunc for routing paths that could
  // not be found. The default 404 handler is `http.NotFound`.