I just emerged kde-apps/cantor-16.04.3 with `-R` in package.use and got the following message: * You have decided to build cantor with no backend. * To have this application functional, please do one of below: * # emerge -va1 '='kde-apps/cantor-16.04.3 with 'analitza', 'lua', 'python', 'qalculate' or 'R' USE flag enabled * # emerge -vaDu sci-mathematics/maxima However, I have octave (version 4.0.3) installed and it is automatically picked up as a back-end. I tested it, and it works. So octave should also be recognized as a possible backend in the ebuild.
Thanks for your report, this was fixed by commit 656ea1f50c74b74e8b31bf4c355c18e612a2bfa6 No need to add a USE flag for an unconditionally available backend, pkg_postinst message was changed to pkg_pretend and now mentions octave as an alternative.