2008-04-15 Ex 2.30 SICP map 使用: (define (square-tree t) (map (lambda (x) (if (pair? x) (square-tree x) (square x))) t)) map 不使用: (define (square-tree t) (cond (null? t) t) ((not (pair? t)) (square t)) (else (cons (square-tree (car t)) (square-tree (cdr t))))))