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