(* a simple test with mutually recursive functions *)
let test =
let rec even = function
| 0 -> true
| n -> odd (n - 1)
and odd = function
| 0 -> false
| n -> even (n - 1)
in
List.iter (fun i -> assert (even i <> odd i && even i = (i mod 2 = 0)))
[0;1;2;3;4;5;6]