(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)))