mux.go
func (mx *Mux) Group(fn func(r Router)) Router {
	im := mx.With().(*Mux)
	if fn != nil {
		fn(im)
	}
	return im
}
func (mx *Mux) Group(fn func(r Router)) Router {
	im := mx.With().(*Mux)
	if fn != nil {
		fn(im)
	}
	return im
}
// Group creates a new inline-Mux with a fresh middleware stack. It's useful
// for a group of handlers along the same routing path that use an additional
// set of middlewares. See _examples/.