お題：自然数nが奇数かどうかチェックする関数oddを定義せよ

c
https://ideone.com/gdiEYq
//int odd(int n) {return n & 1;}
int odd(int);
int even(int n) {return n == 0 ? 1 : odd(n - 1);}
int odd(int n) {return n == 0 ? 0 : even(n - 1);}

ocaml
https://ideone.com/LYsHsF
let rec even = function 0 -> true | n -> odd (n - 1)
and odd = function 0 -> false | n -> even (n - 1)