code: func (mx *Mux) MethodNotAllowed(handlerFn http.HandlerFunc) {
  	// Build MethodNotAllowed handler chain
  	m := mx
  	hFn := handlerFn
  	if mx.inline && mx.parent != nil {
  		m = mx.parent
  		hFn = Chain(mx.middlewares...).HandlerFunc(hFn).ServeHTTP
  	}
  
  	// Update the methodNotAllowedHandler from this point forward
  	m.methodNotAllowedHandler = hFn
  	m.updateSubRoutes(func(subMux *Mux) {
  		if subMux.methodNotAllowedHandler == nil {
  			subMux.MethodNotAllowed(hFn)
  		}
  	})
  }
docs: // MethodNotAllowed sets a custom http.HandlerFunc for routing paths where the
  // method is unresolved. The default handler returns a 405 with an empty body.