Path on another path being subtracted sounds weird to me. I think they are constructed part by part. Deleting a part of a rendered thing, or rendering a path with excluding some parts of it by looking at another path needs twice more rendering time than doing it part by part. Boolean operations on such things are difficult things and I think it is not a solved thing in computer science. I think it is not science at all, it is some sort of pen and paper trick. Computer guys claim that it is possible than they compute it then delete one by one.