other uses in computing (such as with regular expressions) that mean

    something different.}

\end{itemize}

\label{license-compatibility-first-mentioned}

Oddly, this flexibility has received (in the opinion of the authors, undue)

criticism, primarily because of the complex and oft-debated notion of

``license compatibility'' (which is explained in detail in

\S~\ref{license-compatibility}).  Copyleft licenses are generally

incompatible with each other, because the details of how they implement

copyleft differs.  Specifically, copyleft works only because of its

requirement that downstream licensors use the \textit{same} license for

combined and modified works.  As such, software licensed under the terms of

``GPLv2-only'' cannot be combined with works licensed ``GPLv3-or-later''.

This is admittedly a frustrating outcome.

Other copyleft licenses that appeared after GPL, such

as the Creative Commons ``Share Alike'' licenses, the Eclipse Public License

and the Mozilla Public License \textbf{require} all copyright holders choosing

to use any version of those licenses to automatically accept and relicense

their copyrighted works under new versions.  Of course ,Creative Commons, the

Eclipse Foundation, and the Mozilla Foundation (like the FSF) have generally

served as excellent stewards of their licenses.  Copyright holders using

licensing decisions for their copyrights to these organizations without a

second thought.

However, note that FSF gives herein the control of copyright holders to

decide whether or not to implicitly trust the FSF in its work of drafting

future GPL versions.  The FSF, for its part, does encourage copyright holders

to chose by default ``GPLv$X$-or-later'' (where $X$ is the most recent

version of the GPL published by the FSF).  However, the FSF \textbf{does not

  mandate} that a choice to use any GPL requires a copyright holder ceding

its authority for future licensing decisions to the FSF.  In fact, the FSF

considered this possibility for GPLv3 and chose not to do so, instead opting

for the third-party steward designation clause discussed in

Section~\ref{GPLv3s14}.

\section{Complexities of Two Simultaneously Popular Copylefts}

Obviously most GPL advocates would prefer widespread migration to GPLv3, and

many newly formed projects who seek a copyleft license tend to choose a

GPLv3-based license.  However, many existing copylefted projects continue

with GPLv2-only or GPLv2-or-later as their default license.

While GPLv3 introduces many improvements --- many of which were designed to

increase adoption by for-profit companies --- GPLv2 remains a widely used and

extremely popular license.  The GPLv2 is, no doubt, a good and useful