private function getKeys($argv)
    {
        $groupKey = null;
        $key = null;

        if (isset($argv[0])) {
            if ($argv[0] instanceof Group\Key) {
                $groupKey = $argv[0];

                if (isset($argv[1])) {
                    if ($argv[1] instanceof Key) {
                        $key = $argv[1];
                    } else {
                        throw new \InvalidArgumentException("must be instance of Memento_Key");
                    }
                }
            } elseif ($argv[0] instanceof Key) {
                $key = $argv[0];
            } else {
                throw new \InvalidArgumentException("argument 1 must be instance of Memento_Group_Key or Memento_Key");
            }
        }

        return array($key, $groupKey);
    }