what about 

module type OrderedType = sig
      type t
      val compare : t -> t -> int
end


module type S = sig
      type elt
      type t
      type set
      val to_list : t -> elt list
      val to_set : t -> set
end

module Make (Ord : OrderedType) : S = struct
        module EltSet = Set.Make(Ord)
        type elt = EltSet.elt
        type set = EltSet.t
        type t = { l : elt list; s : set }
        let to_list t = t.l
        let to_set t = t.s

end


I'm far to be an expert, but this (or something similar) should work.

p

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