private function visitFactorialNode(FunctionNode $node)
    {
        $functionName = $node->getName();
        $op = $node->getOperand();
        $operand = $op->accept($this);

        // Add parentheses most of the time.
        if ($this->isNumeric($op)) {
            if ($op->getValue() < 0) {
                $operand = "($operand)";
            }
        } elseif ($op instanceof VariableNode || $op instanceof ConstantNode) {
            // Do nothing
        } else {
            $operand = "($operand)";
        }

        return "$operand$functionName";
    }