I think the question is: What if the author has patents A, B and C, but the original code only uses A and B, and you modify it to use patent C as well? I guess

I think the question is: What if the author has patents A, B and C, but the original code only uses A and B, and you modify it to use patent C as well? I guess it doesn't apply for derivative works unless the author incorporates the changes in the distribution of the code.