private static function _compareUsingContentId($r1, $r2)
    {
        $h1 = array_change_key_case($r1->headers);
        $h2 = array_change_key_case($r2->headers);
        $c1 = Utilities::tryGetValue($h1, Resources::CONTENT_ID, 0);
        $c2 = Utilities::tryGetValue($h2, Resources::CONTENT_ID, 0);

        return intval($c1) >= intval($c2);
    }