Explain GPLv2 implicit private modification rights

I've long been aware that GPLv2 "technically" governed private
modifications and that generally there were probably more requirements
on privately modified versions of GPLv2'd works than most people assumed
in practice, including commonly held public interpretation by FSF.

HT Wolvereness, who pointed out to me that GPLv3 solved that problem.
When I spoke to Fontana about it, he was indeed aware that this text was
"missing" in GPLv2 and that GPLv3 properly added it, through some
politics during the GPLv3 process.

I've added herein the ultimate historical conclusions about GPLv2's
interpretation and how GPLv3 clarified it.  I've left out the color
about the politics of how it got added, not because they are not
interesting, relevant and germane to tutorial, but because we don't have
a good place yet in the tutorial for discussion of GPLv3 drafting
politics, and frankly if we have such a section, Fontana ought to write
it, not me.

gpl-lgpl.tex

@@ -1627,6 +1627,38 @@ software alongside proprietary software or other unrelated Free Software,
 as long as the terms of GPL are adhered to for those packages that are
 truly GPL'd.
 
+\subsection{Right to Private Modification} 
+\label{gplv2-private-modification}
+
+The issue of private modifications of GPLv2'd works deserves special
+attention.  While these rights are clearly explicit in GPLv3~\S2\P2 (see
+\~S\ref{GPLv3S2} of this tutorial for details), the permission to create
+private modifications is mostly implicit in GPLv2.  Most notably, the
+requirements of GPLv2~\S2 (and GPLv2~\S3, which will be discussed next) are
+centered around two different copyright controls: both modification
+\emph{and} distribution.  As such, GPLv2~\S2's requirements need only be met
+when a modified version is distributed; one need not follow them for modified
+versions that are not distributed\footnote{As a matter of best practice, it's
+  useful to assume that all software may eventually be distributed later,
+  even if there no plans for distribution at this time.  Too often, GPL
+  violations occur because of a late distribution decision of software that
+  was otherwise never intended for distribution.}.
+
+However, the careful reader of GPLv2 will notice that, unlike GPLv3, no other
+clauses of the license actually give explicit permission to make private
+modifications.  Since modification of software is a control governed by
+copyright, a modifier needs permission from the copyright holder to engage in
+that activity.
+
+In practice, however, traditional GPLv2 interpretation has always assumed
+that blanket permission to create non-distributed modified versions was
+available, and the
+\href{http://www.gnu.org/licenses/gpl-faq.html#GPLRequireSourcePostedPublic}{FSF
+  has long opined that distribution of modified versions is never mandatory}.
+This issue is one of many where GPLv3 clarifies in explicit text the implicit
+policy and intent that was solidified via long-standing interpretation of
+GPLv2.
+
 \section{GPLv2~\S3: Producing Binaries}
 \label{GPLv2s3}
 
@@ -2559,8 +2591,9 @@ permitted without limitation and activities that trigger additional
 requirements.  Specifically, GPLv3~\S2\P2 guarantees the basic freedoms of
 privately modifying and running the program.  While these basic freedoms were
 generally considered a standard part of users' rights under GPLv2 as well,
-the GPLv3 states them herein more explicitly.  (In other words, there is no
-direct analog to the first sentence of GPLv3~\S2\P2 in GPLv2.)
+the GPLv3 states them herein more explicitly.  In other words, there is no
+direct analog to the first sentence of GPLv3~\S2\P2 in GPLv2
+(See \S~\ref{gplv2-private-modification} of this tutorial for more on this issue.)
 
 Also, GPLv3~\S2\P2 gives an explicit permission for a client to provide a
 copy of its modified software to a contractor exclusively for that contractor