code: func (mx *Mux) Group(fn func(r Router)) Router {
  	im := mx.With().(*Mux)
  	if fn != nil {
  		fn(im)
  	}
  	return im
  }
docs: // 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/.