open Base
open Expect_test_helpers_base;;

print_s [%sexp (Not_found_s [%message "foo"] : exn)]

[%%expect
  {|
(Not_found_s foo)
|}]
;;

Not_found

[%%expect
  {|
Line _, characters _-_:
Error (alert deprecated): Not_found
[2016-09] this element comes from the stdlib distributed with OCaml.
Instead of raising [Not_found], consider using [raise_s] with an informative error
message.  If code needs to distinguish [Not_found] from other exceptions, please change
it to handle both [Not_found] and [Not_found_s].  Then, instead of raising [Not_found],
raise [Not_found_s] with an informative error message.
|}]