Let us be lazy and use ocaml -rectypes:

		let fix = (fun f' -> (fun f -> (fun x -> f' (f f) x)) (fun f -> (fun x -> f' (f f) x)) );;
		val fix : (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b = <fun>
		let fact = fix fact' in fact 5;;
		- : int = 120