> or 'C-M-SPC'.

>
> Overriding mark-sexp?
>

Yes, IMO.  I can imagine this list has a preponderance of people that would
miss the binding, though.  Personally, even though I have always written
more elisp than the people immediately around me, I'm a C/C++ programmer,
and mark-sexp doesn't do anything intuitively useful for me in c++-mode, so
I've never internalized it.

Even though I voted for C-x SPC, I would miss gud-break.  Lots of muscle
memory around that, although not as much as Alt-Space. I suppose the M-[
and M-] suggstions aren't so bad.

Oooh.  s-SPC is free.  I shall bind it to s-SPC in my own .emacs anyhow.