Exercise 2.31.  Abstract your answer to exercise 2.30 to produce a 
procedure tree-map with the property that square-tree could be defined 
as

(define (square-tree tree) (tree-map square tree))

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