ParseParens.cs

// Parenthesis?
if (_tokenizer.Token == Token.OpenParens)
{
    // Skip '('
    _tokenizer.NextToken();

    // Parse a top-level expression
    var node = ParseAddSubtract();

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

    // Return
    return node;
}