let () = match GPL with
  | GPL | LGPL ->
     assert
       (let s = "Do not forget to add new constructors to the data list above." in
        Format.ifprintf Format.std_formatter "%s" s;
        true)

Sorry could be much better like this:

let () = match GPL with GPL | LGPL ->
  ignore "Do not forget to add new constructors to the data list above."

--
Julien