;不难看出make-joint需要做的是在客户账户中添加一条额外的密钥并返回该账户
;由于帐户这个对象在系统中是以过程形式定义的，不调用过程无法修改其本地参数
;有必要在make-account中添加额外的命令来收集新密钥

(define (make-account balance password);初始化时以list形式输入password
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define (in? p password1)
          (cond ((null? password1) #f)
                ((eq? p (car password1)) #t)
                (else (in? p (cdr password1)))))
  (define (addpassword password1)
          (set! password (cons password1 password)))
                
  (define (dispatch p m)
    (if (in? p password);测试p是否在password列表中
        (cond ((eq? m 'withdraw) withdraw)
              ((eq? m 'deposit) deposit)
              ((eq? m 'addpassword) addpassword)
              (else (error "Unknown request -- MAKE-ACCOUNT"
                           m)))
        (error "Incorrect password" m))
    )

  dispatch)
(define (make-joint account oldpassword newpassword)
  (begin ((account oldpassword 'addpassword) newpassword) account))