code: public Object f(Widget w, int i) {
      setElement(w.getElement());
      setIndex(i);
      f(w);
      return null;
    }
docs: Override this for GQuery methods which loop over matched widgets and
  invoke a callback on each widget.
  
  NOTE: If your query has non-widget elements you might need to override
  'public void f()' or 'public void f(Element e)' to handle these elements and
  avoid a runtime exception.