>
>
>
> Well, the problem is, that the ocaml type inference forbids a function to
> return a polymorphic value, so the type of make_mem' is only ('_a -> '_b)
> -> '_a -> '_b. So the right thing to do here (as this type is, obviously,
> incorrect) is to use Obj.magic:



I'm sorry, the OCaml  type inference is correct to infer the type ('_a ->
'_b) -> '_a -> '_b here, but I believe that there can be no error caused
generalizing it.