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2. Copying

This document may be redistributed, verbatim or in modified form, under the terms of the GNU General Public License, version 2 or any later version. The same terms apply to the libraries it documents. A copy of the General Public License is provided as an Appendix.

Most XEmacs documentation has its own license, which is an ancestor of the GNU Free Documentation License (FDL), and whose terms are quite similar to those imposed by GNU on Emacs documentation. Why is this manual licensed differently (under the GNU General Public License, or GPL), and why does it have to be distributed separately from the XEmacs User's Guide and the XEmacs Lisp Reference Manual?

Taking the second question first, XEmacs is community-owned software. That is, unlike GNU Emacs, there is no monopoly copyright holder. Many of us, including the original Lucid authors, have contributed our copyrights to the Free Software Foundation (FSF), and of course much content is derived from GNU Emacs, and therefore is held by the FSF. Another large chunk is held by Sun Microsystems, and a few individual authors hold copyright to thousands of lines each. But many individuals hold copyright to only a few dozen lines. Like the Linux kernel, copyright ownership is distributed throughout a community.

However, its license is "copyleft," i.e., it requires that you redistribute it under terms identical to those under which you received it, unless you have explicit permission of the copyright holder. Because of the multiple owners, determining the ownership of any given part of XEmacs is tedious, and perhaps impossible. For practical purposes, then, the license of any substantial chunk of existing XEmacs content cannot be changed, except to a later version of the GPL, for those parts under GPL. (That is due to the explicit permission to change to a later version of the GPL, present in every file of XEmacs.)

Unfortunately, this severe restriction means that the GPL, FDL, and the XEmacs documentation license (XDL) are mutually incompatible. That is, content licensed under any of the GPL, FDL, or XDL may not be mixed with content licensed under either of the other two without changing the license of some of the content. But this requires permission of the copyright holder, which is often difficult or impossible to get.

For example, you may not take comments or docstrings from XEmacs code and add them to the Lispref to mend a gap in the latter's coverage. You may not copy text from the Lispref into docstrings in the code. And you may not copy text from the GNU Emacs Lisp Reference to the XEmacs Lisp Reference Manual. (In this case it is at least trivial to ask permission, although it is rather unclear whether it would be granted.)

In fact, parts of this document were derived by copying from XEmacs code under the GPL, without any further permission from the authors. Thus, this document must be distributed under the GPL, as a "volume" separate from the XEmacs documentation under the XDL. Note that the "mere aggregation" clauses allow us to distribute in the same tarball. But incorporating it as a node in the Lispref is prohibited, even if done by inclusion.

A bit of advocacy:

If you look carefully at the additional restrictions imposed by the soi-disant "free" documentation licenses, you discover that they are simply proprietary restrictions guaranteeing a certain amount of unpaid political advertising to the Free Software Foundation and GNU Project (and in the case of the FDL, this is extended to commercial advertising by authors of original or derived works). Whether this is "ethically justified" or not is a difficult question. What is certain is that there is little social benefit to these terms (since the license documents themselves contain the advocacy and must be included with any distribution).

I conclude it makes sense for XEmacs to reduce its restrictions, where possible, to the "least common denominator," the GNU General Public License.

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