> Hmm, well, what if we change line_parallel() so that what it tests
> is whether (l1->A * l2->B - l2->A * l1->B) is zero? That seems
> simpler and more symmetric than what it does now.
I was thinking to do so. I would also check for both sides not to
overflow, but that can wait. Patch attached. Unfortunately,
the change causes too much noise on the regression tests.