; Aufgabe 6-3 - Bankkonten
(define (make-account balance password)
 (let ((errors 0))
  (define (get-balance) balance)
  (define (withdraw amount)
	(cond ((< balance amount) (error "Error: Insufficient funds!"))
		  (else (begin (set! balance (- balance amount)) balance))))
  (define (deposit amount)
	(set! balance (+ balance amount)) balance)
  (define (change-pw newpass)
    (set! password newpass) (display "Password changed!") (newline))
  (define (wrong-password . dummy)
    (if (< errors 3)
	    (begin (set! errors (+ errors 1))
		       (display "Error: Wrong password! (")
			   (display (- 3 errors)) (display " tries remaining)")
			   (newline))
		(call-police)))
  (define (call-police) (display "Dade county police! Freeze!") (newline))
  (define (dispatch p m)
		(cond ((eq? p password)
 		       (set! errors 0)
 			   (cond ((eq? m 'withdraw) withdraw)
 			 		 ((eq? m 'deposit) deposit)
 					 ((eq? m 'change-password) change-pw)
					 ((eq? m 'get-balance) get-balance)
 					 (else (error "Error: Unknown request to make-account: " m))))
		      (else wrong-password)))
  dispatch))

 (define (make-special-account account pw second-pw)
  (if (number? ((account pw 'withdraw) 0)) ; hack for "password ok"
   (let ((help (cond ((eq? pw 'wrong-password) 'new-wrong-password)
				     (else 'wrong-password))))
	(define (dispatch passwd m)
	 (account (if (eq? passwd second-pw) pw
			                             help)
	          m))
	dispatch)))

(define konto (make-account 10 'sesam))
(define konto2 (make-special-account konto 'sesam 'neues-passwort))