protected static function pushStack(&$context, $operation, $vars) {
        list($levels, $spvar, $var) = Expression::analyze($context, $vars[0]);
        $context['stack'][] = $context['currentToken'][Token::POS_INNERTAG];
        $context['stack'][] = Expression::toString($levels, $spvar, $var);
        $context['stack'][] = $operation;
        $context['level']++;
    }