let insert l a =
    if List.mem_assoc a l
    then
      let n = List.assoc a l in (a, n+1)::(List.remove_assoc a l)
    else (a, 1)::l
;;
let for_each str f =
  let l = MultiSet.empty in
  let rec helper spot final =
    if spot = final then l
    else begin
      f l str.[spot];
      helper (spot + 1) final
    end
  in
  helper 0 (String.length str);