private Iterator<XSComponent> descendants(XSModelGroup mg){
  List<XSComponent> r=new ArrayList<XSComponent>();
  visit(mg,r);
  return r.iterator();
}

private Iterator < XSComponent > descendants ( XSModelGroup mg ) { List < XSComponent > r = new ArrayList < XSComponent > ( ) ; visit ( mg , r ) ; return r . iterator ( ) ; }
iterate all descendant model groups of the given model group , including itself .
