Exercise 2.27.  Modify your reverse procedure of exercise 2.18 to 
produce a deep-reverse procedure that takes a list as argument and 
returns as its value the list with its elements reversed and with all 
sublists deep-reversed as well. For example,

(define x (list (list 1 2) (list 3 4)))

x
((1 2) (3 4))

(reverse x)
((3 4) (1 2))

(deep-reverse x)
((4 3) (2 1))

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(define (deep-reverse x)
  (define (go in out)
    (if (null? in)
        out
        (go (cdr in) (cons (deep-reverse (car in)) out))))
  (if (pair? x)
      (go x nil)
      x))