module IP = struct
   type t = int * string
   let compare x y = let (i, _) = x and (j, _) = y in compare i j
end;;

module PS = Set.Make(IP);;
let ss = ref PS.empty;; 
for i=0 to 10 do ss := PS.add (i, string_of_int i) !ss done;;
PS.elements !ss;;
(* - : PS.elt list =
[(0, "0"); (1, "1"); (2, "2"); (3, "3"); (4, "4"); (5, "5"); (6, "6");
 (7, "7"); (8, "8"); (9, "9"); (10, "10")] *)
PS.elements (PS.add (3, "trois") (PS.singleton (3, "three")));;
(* - : PS.elt list = [(3, "three")] (* senority! *) *)