private function get_group_member_attr($object_classes = array(), $default = 'member')
    {
        if (empty($object_classes)) {
            $object_classes = $this->prop['groups']['object_classes'];
        }

        if (!empty($object_classes)) {
            foreach ((array)$object_classes as $oc) {
                if ($attr = $this->group_types[strtolower($oc)]) {
                    return $attr;
                }
            }
        }

        if (!empty($this->prop['groups']['member_attr'])) {
            return $this->prop['groups']['member_attr'];
        }

        return $default;
    }