Exercise 2.30.  Define a procedure square-tree analogous to the 
square-list procedure of exercise 2.21. That is, square-list should 
behave as follows:

(square-tree
 (list 1
       (list 2 (list 3 4) 5)
       (list 6 7)))
(1 (4 (9 16) 25) (36 49))

Define square-tree both directly (i.e., without using any higher-order 
procedures) and also by using map and recursion.

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(define (square-tree tree)
  (if (pair? tree)
      (cons (square-tree (car tree))
            (square-tree (cdr tree)))
      (square tree)))

(define (square-tree tree)
  (map (lambda (sub-tree)
          (if (pair? sub-tree)
              (square-tree sub-tree)
              (square sub-tree)))
       tree))