(defun set0-helper (e r)
  (cond ((endp r) nil)
        ((set-memberp e (car r))
         (cons (car(car r)) (cons (cdr(car r)) (set0-helper e (cdr r)))))
        (t (set0-helper e (cdr r)))))
 
 
(defun set0 (r s)
  (cons (set0-helper (car s) r) (set0-helper (cdr s) r)))