lib/Twig/lib/Twig/ExpressionParser.php
protected function checkConstantExpression(Twig_NodeInterface $node)
    {
        if (!($node instanceof Twig_Node_Expression_Constant || $node instanceof Twig_Node_Expression_Array
            || $node instanceof Twig_Node_Expression_Unary_Neg || $node instanceof Twig_Node_Expression_Unary_Pos
        )) {
            return false;
        }

        foreach ($node as $n) {
            if (!$this->checkConstantExpression($n)) {
                return false;
            }
        }

        return true;
    }
protected function checkConstantExpression(Twig_NodeInterface $node)
    {
        if (!($node instanceof Twig_Node_Expression_Constant || $node instanceof Twig_Node_Expression_Array
            || $node instanceof Twig_Node_Expression_Unary_Neg || $node instanceof Twig_Node_Expression_Unary_Pos
        )) {
            return false;
        }

        foreach ($node as $n) {
            if (!$this->checkConstantExpression($n)) {
                return false;
            }
        }

        return true;
    }
checks that the node only contains "constant" elements