code: public void f(Widget w) {
      setElement(w.getElement());
      if (loop) {
        loop = false;
        f();
      } else {
        f(w.getElement().<com.google.gwt.dom.client.Element> cast());
      }
    }
docs: Override this for GQuery methods which take a callback, but do not expect a
  return value, apply to a single 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.