let set_member eq x s = 
  List.exists (fun y -> eq x y) s

let rec set_diff eq s1 s2 =
  match s1 with
    [] -> []
  |	x::rest ->
      if (set_member eq x s2)
      then set_diff eq rest s2
      else x::(set_diff eq rest s2)

module type Eq =
  sig
    type t
    val eq : t -> t -> bool
  end

module Set =
  functor (Elt: Eq) ->
  struct
    type element = Elt.t
    type set = element list
    let empty = []
    let member x s = List.exists (fun y -> Elt.eq x y) s
    let add x s = if (member x s) then s else x::s
    let rec union s1 s2 =
      match s1 with
	[] -> s2
      |	x::rest ->
	  if (member x s2) 
	  then union rest s2
	  else x::(union rest s2)
    let rec inter s1 s2 =
      match s1 with
	[] -> []
      |	x::rest ->
	  if (member x s2) 
	  then x::(inter rest s2)
	  else inter rest s2
    let rec subset s1 s2 = 
      match s1 with
	[] -> true
      |	x::rest ->
	  member x s2 && subset rest s2
    let rec diff s1 s2 =
      match s1 with
	[] -> []
      |	x::rest ->
	  if (member x s2)
	  then diff rest s2
	  else x::(diff rest s2)
    let rec uniquify s =
      match s with
	[] -> []
      |	x::rest ->
	  if (member x rest) then rest
	  else x::(uniquify rest)
  end