I'm actually surprised by the lack of rigor in many of the sources I checked. It's obvious that b != 0 is necessary because otherwise you can't get results like line segments having an infinite number of points, and hence equal to the amount of points in a line.[1] The thing is, a lot of common references seem to leave out that the endpoints can't be equal.[2] It's so obvious that it's just implied, but it's dangerous to treat mathematics like that!
