Somwhat complex use of objects in OCaml

objects.ml

let rec non_empty_list next prev value =
  object (self)
    val mutable next = next
    val mutable prev = prev
    val mutable value = value

    method append value =
      let l = non_empty_list next (Some self) value in
      next <- Some l

    method prepend value =
      let l = non_empty_list (Some self) prev value in
      prev <- Some l

    method to_list =
      match prev with
      | None -> self#to_list'
      | Some prev -> prev#to_list

    method to_list' =
      match next with
      | None -> [value]
      | Some next -> value :: next#to_list'
  end

let () =
  let o = non_empty_list None None 2 in
  o#append 4;
  o#append 6;
  o#prepend 0;
  o#to_list |> List.iter print_int;

  print_endline "\n";

  let o = non_empty_list None None "one" in
  o#append "two";
  o#append "three";
  o#prepend "four";
  o#to_list |> List.iter print_endline;

Outputs:

0264

four
one
three
two