Sort groups in topic descending by name

[Nikolaus Rath]

Hello,

I would like to sort the groups in one topic by descending name, and in
another topic by ascending name.

It sounds simply, but I just can't figure it out.

G S a in the group buffer always sorts all groups, even if I first set
the process mark on one topic.

C-u G S a sorts by descending name, but the documentation says this
should happen when I give it a symbolic, rather than numeric prefix.

M-i is supposed to give me a symbolic prefix, but is actually bound to
tab-to-tab-stop.

G P a does, as far as I can tell, nothing at all, no matter if I mark
some groups and if I give it a prefix.

Can someone give me a hint?


Thanks!
-Nikolaus
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