public static function instance( $className = null )
    {
        if ( is_null( $className ) )
        {
            $ini = eZINI::instance( 'ezcomments.ini' );
            $className = $ini->variable( 'NotificationSettings', 'NotificationManagerClass' );
        }
        if ( !isset( self::$instance ) )
        {
            self::$instance = new $className();
        }
        return self::$instance;
    }