SICP 問題2.25

(1 3 (5 7) 9)
(car (cdr (car (cdr (cdr)))))

((7))
(car (car ))

(1 (2 (3 (4 (5 (6 7))))))
(car (cdr (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr ))))))))))))

gosh> (car (cdr (car (cdr (cdr '(1 3 (5 7) 9))))))
7
gosh> (car (car '((7))))
7
gosh> (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr '(1 (2 (3 (4 (5 (6 7))))))))))))))))))
7