* Alain Frisch:

> Why not:
>
> let unwind_protect f g =
>   match (try f (); None with exn -> Some exn) with
>    | None -> g ()
>    | Some exn -> g (); raise exn
>
> ?

unwind_protect traditionally returns the value of f (), not that of
g ().  I think it is fundamentally non-tail-call.