Following statement copied from http://x11-basic.sourceforge.net/X11-Basic-manual.pdf (18.10.2019):
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Copyright (C) 1997-2018 by Markus Hoffmann
Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.2 or any later version
published by the Free Software Foundation; with no Invariant Sections, no FrontCover Texts, and no Back-Cover Texts. A copy of the license is included in the
section entitled "GNU Free Documentation License".
X11-Basic is free software; you can redistribute it and/or modify it under the
terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
for more details.
Read the file COPYING for details.
(Basically that means, free, open source, use and modify as you like, don’t
incorporate it into non-free software, no warranty of any sort, don’t blame me if it
doesn’t work.)