router.go
func (r *Router) URLFor(name string, pairs ...string) string {
	leaf, ok := r.namedRoutes[name]
	if !ok {
		panic("route with given name does not exists: " + name)
	}
	return leaf.URLPath(pairs...)
}
func (r *Router) URLFor(name string, pairs ...string) string {
	leaf, ok := r.namedRoutes[name]
	if !ok {
		panic("route with given name does not exists: " + name)
	}
	return leaf.URLPath(pairs...)
}
// URLFor builds path part of URL by given pair values.