[rer0tsaz]

Just use n = max(a, b). Of course that doesn't help you much unless you're adhering to strict safety standards (e.g. power of ten rules), it just shifts the burden of proof from termination to correctness. And there's no such trick with the Ackermann function.

I'm really just advocating a rule of least power. Don't go into full general recursion just because you can.