Exercise 2.25. Give combinations of cars and cdrs that will pick 7 from each of the following lists:
(1 3 (5 7) 9)
((7))
(1 (2 (3 (4 (5 (6 7))))))
box and pointer
TODO
取出 7
(list 1 (list 2 (list 3 4)))
; (1 (2 (3 4)))
; grab 7
(cadr (caddr '(1 3 (5 7) 9)))
(car (car '((7))))
(cadr (cadr (cadr (cadr (cadr (cadr '(1 (2 (3 (4 (5 (6 7))))))))))))