private function _addTocSubEntry($depth, $level, $title, $identifier, &$list)
    {
        if (!isset($list['sub']))
            $list['sub'] = array();
        $offset = count($list['sub']);
        if ($depth === 1) {
            $list['sub'][$offset] = array(
                'id' => $identifier,
                'value' => $title
            );
            return;
        }
        $offset--;
        if (!isset($list['sub'][$offset]))
            $list['sub'][$offset] = array();
        $this->_addTocSubEntry($depth - 1, $level, $title, $identifier, $list['sub'][$offset]);
    }