let rec lrepeat f xs =
  let rec aux = function
      (LCons(x, xf), i, 0) -> aux(xf(), i+1, (f (i + 1)))
    | (LCons(x, xf) as xl, i, r) -> LCons(x, fun() -> aux(xl, i, (r - 1)))
    | (LNil, _, _) -> LNil
  in aux(xs, 0, (f 0));;

let sublist xs lxs =
  let rec iter = function
      (_, LNil, _) -> LNil
    | ([], lxs, _) -> lxs
    | (h::t, LCons(x, xf), i) -> if h = i
                                 then iter(t, xf(), i+1)
                                 else LCons(x, function() -> iter(h::t, xf(), i+1))
  in iter(List.sort compare xs, lxs, 0);;