public static function subexpression($expression, &$context) {
        $context['usedFeature']['subexp']++;
        $vars = static::analyze(substr($expression, 1, -1), $context);
        $avars = static::advancedVariable($vars, $context, $expression);
        if (isset($avars[0][0]) && !$context['flags']['exhlp']) {
            if (!Validator::helper($context, $avars, true)) {
                $context['error'][] = "Can not find custom helper function defination {$avars[0][0]}() !";
            }
        }
        return array(static::SUBEXP, $avars, $expression);
    }