public static function getMissingExtensionIds($extensions) {
        $inList = array();
        foreach ($extensions as $ext) {
            $inList[] = $ext->getId();
        }
        $missing = array();
        foreach ($extensions as $ext) {
            foreach ($ext->getDependencies() as $extId => $version) {
                if (!in_array($extId, $inList) && !in_array($extId, $missing)) {
                    $missing[] = $extId; 
                }
            }
        }
        return $missing;
    }