[Groff] A licensing question for forking GNU troff

[Steffen Nurpmeso]

Hello,

i am in the process of forking an old version of GNU troff,
starting from the last commit that is still GPL2 licensed
(1.19.2-574-gecbf4f1).  It is yet nothing but a bubble full of
air, of course.  (It will be S-roff.)

I planned to place all completely new code (like file_case) under
the ISC license instead of the GPL2, which seems to be possible
according to GPL2, is this understood correctly?  So whereas
S-roff has to be GPL2 licensed because it is a fork of GNU troff
v1.19.2 such new parts, even though deeply embedded into the
machinery, can be placed under the ISC license?  S-roff will be
a heavily stripped down version with a completely different build
system, with heavily changed manuals etc., but still this
condition is true?

I'm also a bit uncertain because of the thread of Ingo and
Bernd today, what licensing issues exist with small modifications
i apply here and there?  I want to place my own copyright header
in all files of course, but i would like my changes to be covered
by ISC instead of GPL2 -- i would be fine with Public Domain for
that (see next paragraph), but i'm a bit afraid of missing
disclaimers for these parts of the code then, so would it be ok to
say something like "changes copyright [ISC license]"?

And then, should a future GNU troff maintainer like some of the
modifications i've implemented, will reintegration be possible
when the fork is instrumented like that?  Or does GNU still need
a special Public Domain clause?  Can i simply extend the mentioned
"changes" accordingly?

Thanks for any advise.

--steffen