(defun set-member (e set) (cond ((null set) nil) ((eq e (car set)) t) (t (set-member e (cdr set))))) (defun set+ (a b) (cond ((null a) b) ((set-member (car a) b) (set+ (cdr a) b)) (t (cons (car a) (set+ (cdr a) b))))) (defun set* (a b) (cond ((null a) nil) ((set-member (car a) b) (cons (car a) (set* (cdr a) b))) (t (set* (cdr a) b)))) (defun set- (a b) (cond ((null a) nil) ((set-member (car a) b) (set- (cdr a) b)) (t (cons (car a) (set- (cdr a) b)))))