let rec oxford (l : string list) : string = match l with | [] -> "" | x :: [] -> x | x :: y :: [] -> x ^ ", and " ^ y | x :: tl -> x ^ ", " ^ oxford tl let () = print_endline @@ oxford []; print_endline @@ oxford ["a"]; print_endline @@ oxford ["a"; "b"]; print_endline @@ oxford ["a"; "b"; "c"]