ParseVariableOrFunctionCall.cs

// Variable
if (_tokenizer.Token == Token.Identifier)
{
    // Capture the name and skip it
    var name = _tokenizer.Identifier;
    _tokenizer.NextToken();

    // Parens indicate a function call, otherwise just a variable
    if (_tokenizer.Token != Token.OpenParens)
    {
        // Variable
        return new NodeVariable(name);
    }
    else
    {
        // Function call

        // Skip parens
        _tokenizer.NextToken();

        // Parse arguments
        var arguments = new List<Node>();
        while (true)
            {
            // Parse argument and add to list
            arguments.Add(ParseAddSubtract());

            // Is there another argument?
            if (_tokenizer.Token == Token.Comma)
            {
            _tokenizer.NextToken();
            continue;
            }

            // Get out
            break;
        }

        // Check and skip ')'
        if (_tokenizer.Token != Token.CloseParens)
        throw new SyntaxException("Missing close parenthesis");
        _tokenizer.NextToken();

        // Create the function call node
        return new NodeFunctionCall(name, arguments.ToArray());
    }
}