(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)))