type 'a ts = {
    mutable data : 'a;
    mutable prev : 'a ts;
    mutable next : 'a ts;
  };;
      
let rec single elem =
  let ret = {
    data = elem; 
    next = Obj.magic 0;
    prev = Obj.magic 0;
  } in
  ret.next <- ret;
  ret.prev <- ret;
  ret
;;

let rec add elem ({next = n} as lst) =
  let newnext = {
    data = elem;
    prev = lst;
    next = n
  } in
  n.prev <- newnext;
  lst.next <- newnext
;;

let l = single (0, 0);;
    
let rec adder cnt =
  add (cnt, 0) l;
  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.data in
  iterator (a + i) (cnt - 1) alst.next
;;
    
let rec test cnt =
  ignore (iterator 0 1000 l);
  if cnt = 0 then () else test (cnt - 1)
;;

test 1000000;;