ParseUnary.cs

// Parse a unary operator (eg: negative/positive)
Node ParseUnary()
{
    // Positive operator is a no-op so just skip it
    if (_tokenizer.Token == Token.Add)
    {
        // Skip
        _tokenizer.NextToken();
        return ParseUnary();
    }

    // Negative operator
    if (_tokenizer.Token == Token.Subtract)
    {
        // Skip
        _tokenizer.NextToken();

        // Parse RHS 
        // Note this recurses to self to support negative of a negative
        var rhs = ParseUnary();

        // Create unary node
        return new NodeUnary(rhs, (a) => -a);
    }

    // No positive/negative operator so parse a leaf node
    return ParseLeaf();
}