That's something else: x == y, so the difference must be 0. What you said above is if x != y, x - y != 0 which holds for all x and y, but is different to

That's something else: x == y, so the difference must be 0. What you said above is if x != y, x - y != 0 which holds for all x and y, but is different to if dx != 0, x + dx != x which fails for some (many!) x and dx.