code: func (r *Route) URLPath(pairs ...string) (*url.URL, error) {
  	if r.err != nil {
  		return nil, r.err
  	}
  	if r.regexp.path == nil {
  		return nil, errors.New("mux: route doesn't have a path")
  	}
  	values, err := r.prepareVars(pairs...)
  	if err != nil {
  		return nil, err
  	}
  	path, err := r.regexp.path.url(values)
  	if err != nil {
  		return nil, err
  	}
  	return &url.URL{
  		Path: path,
  	}, nil
  }
docs: // URLPath builds the path part of the URL for a route. See Route.URL().
  //
  // The route must have a path defined.