問題3.3

(define (make-account balance password)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define (dispatch pw m)
    (begin (unless (eq? pw password) (error "Incorrect password"))
           (cond ((eq? m 'withdraw) withdraw)
                 ((eq? m 'deposit) deposit)
                 (else (error "Unknown request -- MAKE-ACCOUNT" m)))))
  dispatch)