public static function exportList($xml)
    {
        $categories = PermissionKeyCategory::getList();
        $pxml = $xml->addChild('permissionkeys');
        foreach ($categories as $cat) {
            $permissions = static::getList($cat->getPermissionKeyCategoryHandle());
            foreach ($permissions as $p) {
                $p->export($pxml);
            }
        }
    }