I think I'm using Gnus quite wrong, because I cannot get it to
auto-expire old articles (which I have read or marked as read with using
'd' in the summary buffer) from mailing lists.  For hunting down the
problem, I have two questions:

1. Can I find out somehow if the list (e.g. nnml+private:mail.maxima) is
   correctly recognized as auto-expirable?

2. I observe that a mail which I mark with 'd' is marked by 'r' (but not
   automatically by 'E' as expired).  When I visit the buffer later I
   see it marked with 'O'.  Is this a sign of a bad setup?