题目的例子等价于：

(let ((x 3))  (let ((y (+ x 2)))    (let ((z (+ x y 5)))      (* x z))))

于是代码如下：

(define (make-let bindings body)    (cons 'let (cons bindings body)))(define (let*->nested-lets exp)  (define (expand-let* bindings body)    (if (null? bindings)        body        (list (make-let (list (car bindings))                        (expand-let* (cdr bindings) body)))))  (car (expand-let* (cadr exp) (cddr exp))))