sclass WithURL<A> extends Var<A> {
  S url;
  
  *() {}
  *(S *url, A value) { super(value); }
  *(A value) { super(value); }
  
  S url() { ret url; }
  
  toString {
    ret super.toString() + (empty(url) ? "" : " (" + url + ")");
  }
}