Those who need the 1-type-variable polymorphic set data structure I've 
talked of recently will find this file useful :

http://www.polytechnique.fr/eleves/1996/boursetty/polyset.ml

!! Warning !! The URL will stop being effective at the beginning of July.
Would anyone download this very small file to a more persistent website ?

It is a small modification to the original set.ml

In fact, only type declarations are modified, the code is just the same. 
Doesn't this mean that the compiler polymorphism handling could be
enhanced ? 

In the case of sets I feel like declaring something like

(* This is not valid o'caml *)
module type 'a Comparison =
  sig
    val compare : 'a -> 'a -> int
  end

and a functor from modules with type 'a Comparison to modules with type

(* Neither is this *)
module type 'a Set =
  sig
    type 'a t
    val empty: 'a t
    val is_empty: 'a t
    val mem: 'a -> 'a t -> bool
    val add: 'a -> 'a t -> 'a t
  .
  .
  .
  end

That is only my personal, programmer's point of view. I don't know of the
typing problems behind it.

Benoit de Boursetty