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']++;
}