public static function compileSubExpression(&$context, $vars) {
        $ret = static::customHelper($context, $vars, true, true, true);

        if (($ret === null) && $context['flags']['lambda']) {
            $ret = static::compileVariable($context, $vars, true, true);
        }

        return array($ret ? $ret : '', 'FIXME: $subExpression');
    }