type ('a, 'b) t =
  | F of ('a -> 'b)
  | O : ('c, 'b) t * ('a, 'c) t -> ('a, 'b) t

let (@) g f = fun x -> x |> f |> g

let rec compute (f : ('a, 'b) t) : 'a -> 'b = (* ... The type constructor c#29 would escace its scope *)
  match f with
  | F f1 -> f1
  | O (f1, f2) ->
    let f1' : 'c -> 'b = compute f1 in
    let f2' : 'a -> 'c = compute f2 in
    f1' @ f2'