(define make-account
  (lambda args
    (let ((balance
	   (if (null? args)
	       100
	       (car args))))
      (lambda (message)
	(cond
	 ((eq? message 'withdraw)
	  (lambda (amount)
	    (if (<= amount balance)
		(set! balance (- balance amount)))
	    balance))
	 ((eq? message 'deposit)
	  (lambda (amount)
	    (set! balance (+ balance amount))
	    balance))
	 (else (no-method 'account)))))))

(define make-speaker
  (lambda ()
    (lambda (message)
      (cond ((eq? message 'say)
             (lambda (self stuff)
               (display stuff)
               (newline)))
            (else (no-method 'speaker))))))

(define make-lecturer
  (lambda ()
    (let ((speaker (make-speaker)))
      (lambda (message)
        (cond
         ((eq? message 'lecture)
          (lambda (self stuff)
            (ask self 'say stuff)
            (ask self 'say '(abstraction
                             abstraction
                             abstraction))))
         (else
          (get-method speaker message)))))))

(define make-arrogant-lecturer
  (lambda ()
    (let ((lecturer (make-lecturer)))
      (lambda (message)
        (cond ((eq? message 'say)
               (lambda (self stuff)
                 (ask lecturer 'say
                      (append '(it is obvious that)
                              stuff))))
              (else (get-method lecturer message)))))))