@DavidKButlerUoA You're not going to like it! I'm pretty sure you need some way of comparing numbers or testing for equality.
The answer I had starts with m(x, y, 0) and either give you min(x, y) or x+y. So if you can test for equality you can pick one of m(x, y, x), m(x, y, y) or m(x, y, 0)