(* This uses the option type we discussed in class;
 * the caller should use pattern matching to extract
 * the return value.
 *)
let rec get_nth (xs: string list)(n: int) : string option =
  match xs with
  | z::zs when n = 1 -> Some z
  | z::zs when n > 1 -> get_nth zs (n-1)
  | _ -> None