let chars s =
  let rec aux acc i =
    if i < 0 then acc else aux (s.[i] :: acc) (i - 1) in
  aux [] (String.length s - 1)
let f877 s =
  let rec ccr acc col row = function
    | [] -> acc
    | c::cs when c = '\n' -> ccr ((c, col, row) :: acc) 0 (row + 1) cs
    | c::cs -> ccr ((c, col, row) :: acc) (col + 1) row cs in
  let with_pos cs = List.rev (ccr [] 0 0 cs) in
  let isspace c =
  	c = ' ' || c = '\x0c' || c = '\n' || c = '\r' || c = '\t' || c = '\x0b' in
  let disp (c, col, row) =
    if not (isspace c) then Printf.printf "[%c, %d, %d]\n" c col row in
  List.iter disp (with_pos (chars s))
let () = f877 "a b c\n\nd"
 

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