156: How do I kill all articles in Gnus but those matching a pattern?

  Example kill file code:

    ;; kill everything
    (gnus-kill "subject" "" nil nil)
    ;; then restore stuff by our favorite poster
    (gnus-kill "from" "good-guy"
               (function
                (lambda ()
                  (if (eq ?X (char-after (save-excursion
                                           (beginning-of-line 1)
                                           (point))))
                      (gnus-summary-clear-mark-forward 1))))
               t)