let read_file filename =
  let ch = open_in filename in
  let s = really_input_string ch (in_channel_length ch) in
  close_in ch;
  s
;;

let list_of_chars str =
  List.init (String.length str) (String.get str)
;;

let rec transpose = function
  | []
  | [] :: _ -> []
  | rows -> List.map List.hd rows :: transpose (List.map List.tl rows)
;;