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))))))

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(car (cdr (car (cdr (cdr x)))))

(car (car x))

(car (cdr (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr x))))))))))))