public function loadDescendantProductGroupIDListInto(&$idList)
    {
        if ($children = $this->AllChildren()) {
            foreach ($children as $child) {
                if (in_array($child->ID, $idList)) {
                    continue;
                }

                if ($child instanceof self) {
                    $idList[] = $child->ID;
                    $child->loadDescendantProductGroupIDListInto($idList);
                }
            }
        }
    }