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