type list = Nil | Cons of cell
  and cell = {hd : int; mutable tl : list};;

let build_list f n = (* let's assume that always n > 0 *)
  let cl = { hd = f (); tl = Nil } in
  let res : list = Cons cl in
  let cur = ref cl in
  for i = 2 to n do
    let c = { hd = f (); tl = Nil } in
    (!cur).tl <- Cons c;
    cur := c;
  done;
  res;;