(* naive persistent implementation using a map:
 * a mapping i->j means that repr(i)=repr(j)
 * no mapping for i means repr(i)=i
 *)

module M = Map.Make(struct type t = int let compare = compare end)

type t = int M.t

let create n = M.empty

let find m i = 
  let rec lookup i = try lookup (M.find i m) with Not_found -> i in
  lookup i
    
let union m i j =
  let ri = find m i in
  let rj = find m j in
  if ri <> rj then M.add ri rj m else m