class SimplePanelIterator implements Iterator<Widget> {

 bool hasElement = false;
 Widget returned = null;
 SimplePanel _panel;

 SimplePanelIterator(this._panel) {
   hasElement = this._panel.widget != null;
 }

 /**
  * Returns whether the [Iterator] has elements left.
  */
 bool moveNext() {
   return hasElement;
 }
 
 Widget get current => _getCurrent();

 /**
  * Gets the next element in the iteration. Throws a
  * [StateError] if no element is left.
  */
 Widget _getCurrent() {
   if (!hasElement || (this._panel.widget == null)) {
     throw new Exception("NoSuchElement");
   }
   hasElement = false;
   return (returned = this._panel.widget);
 }

 remove() {
   if (returned != null) {
     _panel.remove(returned);
   }
 }
}