(defun set0 (r s)
  (cond ((endp s) nil)
        ((memberp (car s) (car r)) (cons (car(car r)) 
                                         (cons (cdr(car r)) 
                                               (set0 (cdr r) s))))
        ((not (endp r))
         (memberp (cdr s) (car r)) (cons (car(car r)) 
                                         (cons (cdr(car r)) 
                                               (set0 (cdr r) s))))
         (set0 (cdr r) s)))