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Subject: [Caml-list] recursive module type

Hello,

I would like to define some recursive module type in caml, like
module type S = sig
  ...
  module Toto(M : S) : sig
   ...
  end
end

More precisely, I'd like to extend the Set module available in the ocaml distribution with a function that convert a set into another set.
Intuitively, I came up with the following signature S, which is not accepted by ocaml (Error: Unbound module type S):

   module type S = sig

     include Set.S

     module Convert : functor(SetTarget : S) -> sig
       val convert : (elt -> SetTarget.elt) -> t -> SetTarget.t
     end
   end

Is there a way to define such a module type in ocaml, and if not, is there a theoretical reason for this not being possible?

Thanks,
Michael Lienhardt