SICP 练习2.31 tree-map

(define (tree-map proc tree)  (map (lambda (sub-tree)         (cond ((null? sub-tree) ())               ((not (pair? sub-tree)) (proc sub-tree))               (else (tree-map proc sub-tree))))       tree))(define square  (lambda (x) (* x x)))(define (square-tree tree) (tree-map square tree))(square-tree   (list 1        (list 2 (list 3 4) 5)        (list 6 7)))