(define (make-account balance acc-num)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin
          (set! balance (- balance amount))
          balance)
        "Not enough money"))

  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)

  (let ([balance-serializer (make-serializer)])
    (define (dispatch m)
      (cond
        [(eq? m 'withdraw) withdraw]
        [(eq? m 'deposit) deposit]
        [(eq? m 'serializer) balance-serializer]
        [(eq? m 'balance) balance]
        [(eq? m 'number) acc-num]
        [else (error "Wrong request -- MAKE-ACCOUNT" m)]))
    dispatch))

(define (serialized-exchange account1 account2)
  (let ([serializer1 (account1 'serializer)]
        [serializer2 (account2 'serializer)])
    (when (< (account2 'number) (account1 'number))
      (begin
        (set! serializer1 (account2 'serializer))
        (set! serializer2 (account1 'serializer))))
    ((serializer1 (serializer2 exchange)) account1 account2)))