patree.go
func (r *Route) UseHandler(h Handler) {
	if r.f == nil {
		r.f = h
		return
	}

	route := r.getLeaf()
	route.next = &Route{f: h}
}
func (r *Route) UseHandler(h Handler) {
	if r.f == nil {
		r.f = h
		return
	}

	route := r.getLeaf()
	route.next = &Route{f: h}
}
// UseHandler appends a Handler to the route.