Here, let us use the compiler! For sake of readability, we abbreviate fold and unfold.

let ( ! ) f = fold f and ( @ ) a1 a2 = unfold a1 a2;; let fix = !(fun f' -> !(fun f -> !(fun x -> f' @ (f @ f) @ x)) @ !(fun f -> !(fun x -> f' @ (f @ f) @ x)) );;