Only data files are proprietary. The code is all under the same license as
Cube, which is a license based on the zlib license with an additional clause:
> 4. Source versions may not be "relicensed" under a different license
> without my explicitly written permission.
So, a basic copyleft restriction. I don't know for sure why the author of
Cube added their own copyleft clause to a lax permissive license rather than
just using an existing copyleft license, but either way, it seems to be fine
(other than being GPL-incompatible).
