open List;;
 
let rec flipfold f h acc = function
  | []    -> acc
  | y::ys -> flipfold h f (f y acc) ys;;
 
let flipmap f h = flipfold (fun x a -> (f x)::a) (fun y a -> (h y)::a) [];;
 
let rec wordpath alphabet length =
  if length <= 1 then map (fun x -> [x]) alphabet
  else
    let prep xs x = map (fun y -> x::y) @@ wordpath xs (length - 1) in
    flatten @@ flipmap (prep alphabet) (prep @@ rev alphabet) alphabet;;
 
let rec print_path = function
  | []    -> ()
  | x::xs -> iter print_char x; print_char '\n'; print_path xs;;
 
print_path @@ wordpath ['a'; 'b'; 'c'] 3;;

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