2.25

练习 2.25 给出能够从下面各表中取出 7 的car和cdr组合：

(1 3 (5 7) 9)
((7))
(1 (2 (3 (4 (5 (6 7))))))

(define list1 (list 1 3 (list 5 7) 9))
(define list2 (list (list 7)))
(define list3 (list 1 (list 2 (list 3 (list 4 (list 5 (list 6 7)))))))

list1

list2

list3

(car (cdr (car (cdr (cdr list1)))))

(car (car list2))

(car (cdr (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr list3))))))))))))