I'll re-upload with an added paragraph, but... the file mentions a
special exception and I couldn't find it.
I opened a ticket issue upstream:
https://github.com/janestreet/ppxlib_jane/issues/1
I hope that's good enough...
it would be great if that exception is mentioned somewhere in the
source package.
However, there is already an SPDX short identifier[1] for the OCAML
LGPL with execption and I think it will suffice to add this to your
debian/copyright.