Hi Dirk,
Good to hear that ROOT was useful, thanks for sharing!
Most of ROOT’s own code (i.e. not contributed packages such as cppyy) are LGPL’ed. I’m no lawyer, and so I cannot answer your question. But I have a personal opinion / expectation, and that is that a re-implementation does not depend on the license of the original software, but that we would certainly appreciate a “kudos” in publications / documentation / code!
Out of curiosity, which part are we talking about?
Cheers, Axel.