type bit_set = int

let empty = Int64.zero

let union set1 set2 = Int64.logor set1 set2

let inter set1 set2 = Int64.logand set1 set2

let complement set = Int64.lognot set

let diff set1 set2 = inter set1 (complement set2)

let subset set1 set2 = diff set2 set1 = empty

let equal set1 set2 = set1 = set2

let add x set = 
  union (Int64.shift_left Int64.one x) set

let mem x set = 
  inter (Int64.shift_left Int64.one x) set <> empty

let rec interval a b =
  if a > b then [] else a :: interval (a + 1) b

let elements set =
  List.filter (fun x -> mem x set) (interval 0 63);;

let set1 = add 1 (add 2 (add 3 empty))
and set2 = add 3 (add 4 (add 5 empty));;
elements (union set1 set2);;
elements (diff set1 set2);;
elements (inter set1 set2);;