module type SetSig =
sig
   type elt
   type t
   
   val empty : t
   val add : t -> elt -> t
   val union : t -> t -> t
   val remove : t -> elt -> t
   val mem : t -> elt -> bool
end

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

module MakeSet (Elt: EltSig) : (SetSig with type elt = Elt.t) =
struct
   type elt = Elt.t
   type t = elt list

   let empty = []
   let add t elt = elt::t
   let union = (@)
   let rec remove t elt =
      match t with
         [] ->
            []
       | hd :: tl ->
            let tl' = remove tl elt in
            if Elt.compare hd elt = 0 then 
               tl'
            else
               hd :: tl'
   let rec mem t elt =
      match t with
         [] -> false
       | hd :: tl ->
            (Elt.compare hd elt = 0) || (mem tl elt)
end

module IntElt = struct
   type t = int
   let compare i1 i2 = if i1 > i2 then 1 else if i1 = i2 then 0 else -1
end

module IntSet = MakeSet(IntElt)

open IntSet

let set = add (add empty 1) 2
let foo = if mem set 1 then "OK" else "Bug"