The invariant sections are not forbidden by DFSG

[Anton Zinoviev <anton@lml.bas.bg>]

[In order not to write twice same thing and because this can be of
interest to many developers, I will reply to some of the comments of
Wouter Verhelst and Anthony Towns in this separate thread.]

My thesis is that the invariant sections do not contradict DFSG.
Notice that in this particular email message I do not claim that the
invariant sections do not make the documents non-free.  I only say
that DFSG says nothing about whether the invariant sections make a
document free or non-free.

This is the portion of DFSG that matters here:

   Derived Works

   The license must allow modifications and derived works, and must allow
   them to be distributed under the same terms as the license of the
   original software.

Notice that DFSG do not say "arbitrary modifications".  This is
important because "arbitrary modifications" are not allowed by GPL and
by many other licenses.  For example GPL says

   If the modified program normally reads commands interactively when
   run, you must cause it, when started running for such interactive
   use in the most ordinary way, to print or display an announcement
   including an appropriate copyright notice and a notice that there
   is no warranty (or else, saying that you provide a warranty) and
   that users may redistribute the program under these conditions, and
   telling the user how to view a copy of this License.

It doesn't matter how strong this GPL-restriction is.  The point is
that GPL does not allow arbitrary modifications and that DFSG do not
require arbitrary modifications.

If we want to decide whether some particular restrictions in the
license make the license non-free or not, we have to use external to
DFSG arguments.  For example everybody is free to decide that the
invariant sections make the document non-free but this can not be a
consequence from DFSG.

My personal addition to DFSG is this: the license must allow us to
improve the program and/or the documentation.  I think GFDL is free
because GFDL allows us to improve the document (even the invariant
sections can be improved as I demonstrated in my proposal).  However
you see that my conclusion is not based only on DFSG.  It can not be
based only on DFSG because DFSG say nothing about what modifications
must be allowed by the license.

Anton Zinoviev