Comment (by Roland Haas):

I would note that the non-commercial CC licenses are incompatible with GPL, since GPL compatibility requires that commercial use is allowed. This is not much of an issue for us since we never distribute a derived object, but eg the compiled Cactus executable (or a tarball of the full source code) cannot be distributed if it contains both CC-NC and GPL code since the licenses conflict (but one can still use it internally of course). See https://www.gnu.org/licenses/license-list.en.html#CC-BY-NC

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2551/include-reprimand-in-the-et