I see, so the point you wanted to draw was that if you wanted to implement two functions, mutual recursion might let you separate the implementation logic. That is a nice trick.

> you've changed the semantics and engineering tradeoffs so much that we might as well write `n % 2 == 0`.

That is a fairly glaring weakness of the example you've chosen - it is taking a simple situation and overthinking it. It doesn't really matter because I see your point and surely one would exist, but do you have an example where this technique is an efficient solution?