Practice 4.7

let* 中的变量约束是在同一个环境中完成而嵌套的let每个约束都是新环境，每个后继约束创建的新环境是前一个环境的子环境，它继承了前一个的所有约束，所以嵌套的let最后的body的执行环境和let*的body 的执行环境是等价的

(define (let*->nested-lets exp)
  (let ((bind-list (let*-binds exp))
        (body (let*-body exp)))
    (if (null? bind-list)
        body
        (make-let (car bind-list)
                  (let*->nested-lets (make-let* (cdr bind-list) body))))))

(define (let*? exp)
  (tagged-list? exp 'let*))

(define (let*-binds exp)
  (cadr exp))

(define (let*-body exp)
  (caddr exp))

(define (make-let* bind-list body)
  (list 'let* bind-list body))