(* this is fine according to Warning 52 *)
let second1 l =
  try Some (List.nth l 1) with
  | Failure x -> begin
      match x with
      | "nth" -> None
      | _ -> None
    end

(* but this isn't! *)
let second2 l =
  try Some (List.nth l 1) with
  | Failure "nth" -> None
  | Failure _ -> None