class ['a] lst data = 
  object
    val mutable data = (data : 'a)
    val mutable prev = ((Obj.magic ()) : 'a lst)
    val mutable next = ((Obj.magic ()) : 'a lst)
    method add new_data = 
      new lst new_data in
      new_elem#set_next next;
      new_elem#set_prev (next#get_prev);
      next#set_prev new_elem;
      next <- new_elem
    method get_prev = prev
    method get_next = next
    method set_prev elem = prev <- elem
    method set_next elem = next <- elem
    method get = data
  end
;;
 
let l = new lst (0, 0);;
l#set_next l;;
l#set_prev l;;


let rec adder cnt =
  l#add (cnt, 0);
  if cnt > 0 then adder (cnt - 1)
in adder 1000;;

let rec iterator a cnt alst =
  if cnt = 0 then a else
  let (i, _) = alst#get in
  iterator (a + i) (cnt - 1) alst#get_next
;;
    
let rec test cnt =
  ignore (iterator 0 1000 l);
  if cnt = 0 then () else test (cnt - 1)
;;
  
test 1000000;;