public static function getGroupedItems($a_obj_id, $a_use_subtree_by_id = false)
 {
     static $cache = array();
     if (isset($cache[$a_obj_id])) {
         return $cache[$a_obj_id];
     }
     return $cache[$a_obj_id] = ilLPCollections::getGroupedItems($a_obj_id, $a_use_subtree_by_id);
 }