module type FOO = sig
  type t
  type ('a, 'b) op

  val blah1 : (t, t * t) op
  val blah2 : (t, t list) op
end with type ('a, 'b) op = 'a -> 'b

(* Changing that final = to a := does not work:

Characters 117-145:
  end with type ('a, 'b) op := 'a -> 'b;;
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: Only type constructors with identical parameters can be substituted.

   What does this error mean?

*)