연습문제 4.6

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

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

(define (let-vars exp)
  (let ((var-exp-list (cadr exp)))
    (map car var-exp-list)))

(define (let-exps exp)
  (let ((var-exp-list (cadr exp)))
    (map cadr var-exp-list)))

(define (let->combination exp)
  (cons 
   (make-lambda (let-vars exp)
                (let-body exp))
   (let-exps exp)))