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