How do I kill all articles in GNUS but those matching a pattern?

Example:

  ;; 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-Subject-clear-mark-forward 1))))
             t)