According to the changelog the license switched to GPL and says to check out file 'copyright', but find does not return such a file. There is a copy of the GPL in the html documentation, but it's not clear from the source what exactly the license, or whether any later version may be used. <https://www.gnu.org/licenses/gpl-howto.en.html>
Debian states it is v2 or any later version <https://metadata.ftp-master.debian.org/changelogs//main/b/bisonc++/bisonc++_4.09.02-1_copyright>, and I'm guessing that's likely the intention here as well, but want to clear it up first.
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