该题实现如下：
(define (tree-map pro tree)
(map (lambda (x-tree)
(if (pair? x-tree)
(tree-map pro x-tree)
(pro x-tree)))
tree))
递归调用原则：假设要实现的函数为F，只要满足如下条件就可以在函数体中递归调用F:
1、调用时想要F实现的功能与定义时一致
2、参数类型完全相同
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