gnus-posting-styles: detecting private replies in mailing list groups

[Karl Kleinpaste <karl@charcoal.com>]

Is there a suitable predicate for use in gnus-posting-styles which
would allow one to detect that one is writing a _private_ reply to a
mailing list article?

I'd like to change my Gcc on the basis of this.  Normally, followups
out of any of my nnml:list.* groups are dropped in an outgoing
nnml:list.sent group, but when I'm replying privately, any
correspondence which ensues pretty much belongs in my personal.general
group.  It would be nice if I didn't have to remember to fix this
myself every time.

(message-mail-p) isn't adequate by itself, because both list followups
and private replies are mail in such cases.