let rec take_while f xs = 
  match xs with
  | [] -> []
  | x :: rest -> if f x
                 then x :: take_while f rest
                 else []
 
let rec drop_while f xs = 
  match xs with
  | [] -> []
  | x :: rest -> if f x
                 then drop_while f rest
                 else xs
 
let span f xs = (take_while f xs, drop_while f xs)
 
let string_to_char_list s =
  let rec string_to_char_list_impl i l =
    if i < 0 
    then l 
    else string_to_char_list_impl (i - 1) (s.[i] :: l) in
  string_to_char_list_impl (String.length s - 1) []
 
let char_list_to_string cs = 
  String.concat "" (List.map (fun x -> String.make 1 x) cs)
 
let next_permutation word = 
  let rec next_permutation_impl char_list =
    match char_list with
    | x0 :: rest0 -> 
       (match next_permutation_impl rest0 with
        | (true, rest1) -> (true, x0 :: rest1)
        | (false, rest1) -> 
           (match rest1 with
            | [] -> (false, x0 :: rest1)
            | x1 :: _ ->
               if x0 < x1
               then 
                 let (left, right) = span (fun x -> x > x0) rest1 in
                 let chunk = 
                   (match (List.rev left) with
                    | [] -> List.concat [[x0]; right]
                    | l :: revLeft -> List.concat [[l]; List.rev revLeft; [x0]; right])
                 in (match chunk with
                     | (x2 :: rest2) -> (true, x2 :: List.rev rest2)
                     | [] -> raise (Failure "Fuck"))
               else (false, x0 :: rest1)))
    | [] -> (false, [])
  in
  let char_list = string_to_char_list word in
  match next_permutation_impl char_list with
  | (true, result) -> Some (char_list_to_string result)
  | _ -> None
 

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