protected function render()
    {
        if (!isset($this->items[0])) {
            return '';
        }
        foreach ($this->items as $itemId => $item) {
            $item->setCollapsable(
                self::$id, 
                self::$id . $itemId, 
                in_array($itemId, $this->openedItems));
        }
        $this->setAttribute('id', self::$id);
        $this->addCssClass($this->itemType . '-group');
        return $this
            ->html('<div' . $this->getEltDecorationStr() . '>')
            ->html(implode("\n", $this->items))
            ->html('</div>')
            ->getHtml();
    }