removing read marks in gnus

[Werner Kurator]

I gave Gnus a try yet one thing eludes me, how can Gnus be configured
that whenever I exit a group (mail-archive-group that is in which I
store interesting posts) all _read_ articles should be marked
_unread_ again automatically. 

a key-stroke or some code snippet could help here :-)

Werner Kurator 

Since this is a Newsgroup: please don't Cc me, 
-- 
Emacs has also evolved to break more standards in more ways than any other
non-Microsoft program I can think of; it's held together with spit, duct
tape, and collusion against defenseless compiler vendors.
  -- Peter Seebach (1999)