make-joint 是对 make-account 所产生的对象的一次再包装。
注意因为兼容性的问题， display-wrong-another-password 接受了一个不会用到的参数。
;;; 7-make-joint.scm
(define (make-joint origin-acc origin-password another-password)
(lambda (given-password mode)
(if (eq? given-password another-password)
(origin-acc origin-password mode)
display-wrong-another-password-message)))
(define (display-wrong-another-password-message useless-arg)
(display "Incorrect another password"))
测试：
1 ]=> (load "7-make-joint.scm")
;Loading "7-make-joint.scm"... done
;Value: display-wrong-another-password-message
1 ]=> (load "3-make-account.scm")
;Loading "3-make-account.scm"... done
;Value: make-account
1 ]=> (define jack-acc (make-account 100 'jack-password))
;Value: jack-acc
1 ]=> (define peter-acc (make-joint jack-acc 'jack-password 'peter-password))
;Value: peter-acc
1 ]=> ((peter-acc 'peter-password 'withdraw) 50)
;Value: 50
1 ]=> ((jack-acc 'jack-password 'withdraw) 0)
;Value: 50