let for_each str =

  let rec insert_at_end l c i =
    match c with
    | c when String.length c - 1 > i -> l :: insert_at_end ( insert l (String.get c i))  ( String.sub c 1  (String.length c - 1)  )  (i + 1)
    | c when String.length c - 1 = 0 -> l :: [insert l (String.get c 0)]
  in insert_at_end [] str 0
;;