let () =
  let x = !(ref 2) in
  let y = !(ref 3) in
  let z = !(ref (-4)) in
  assert (x + y = 5);
  assert (x + z = -2);
  assert (z + y = -1);
  assert (x + y + z = 1);
  assert (x - y = -1);
  assert (x - z = 6);
  assert (z - y = -7);
  assert (x + y - z = 9);
  assert (x - y + z = -5);
  assert (x - y - z = 3);

  let start = Unix.gettimeofday () in
  let rec loop x y =
    let x = x - y in
    let x = x - y in
    let x = x - y in
    let x = x - y in
    let x = x - y in
    let x = x - y in
    let x = x - y in
    let x = x - y in
    if x < 1000000000 then
      loop x y
  in
  loop 1 (-1);
  Printf.printf "%f\n" (Unix.gettimeofday () -. start);

  let start = Unix.gettimeofday () in
  let rec loop x y =
    let x = x + y in
    let x = x + y in
    let x = x + y in
    let x = x + y in
    let x = x + y in
    let x = x + y in
    let x = x + y in
    let x = x + y in

    if x < 1000000000 then
      loop x y
  in
  loop 1 1;
  Printf.printf "%f\n" (Unix.gettimeofday () -. start)