(defun intersection-mark (x y)
  (cond ((null x) (do ((c 0 (1+ c))
               (l nil (if (get (nth c y) 'tag)
                  (cons (nth c y) l)
                  l)))
              ((= c 5) l)))
    (t (and (setf (get (car x) 'tag) t)
        (intersection-mark (cdr x) y)))))