bug#68863: Add support for using setf with seq-subseq

[Michael Heerdegen via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hello again,

There is a second point I want to hint at/ discuss:

Some time ago we have removed lots of or even the majority of the
so far implemented generalized variables.

The main criteria were: are they useful, and is the semantics clear.  We
should think about whether this is the case here.

For `seq-subseq' the semantics is not completely unambiguous.  For
example, the setter could exchange a subsequence - but also simply
insert an additional piece, creating a longer sequence.  Both operations
can be useful.

Likewise, when an index (e.g. the starting position) is going beyond the
end of a sequence, Emacs could raise an error, or provide functionality
to append to the existing sequence.  Again, both kinds of behavior could
make sense in different situations.

Finally, the question of whether the operation is destructive, or which
of the involved sequences are reused, is not trivial.

Any thoughts about these points?  Is this generalized variable useful
enough to make this acceptable?  Which kind of implementation do we
prefer then?


Michael.