(define (serialized-exchange account1 account2)
  (let ((serializer (account1 'serializer))
        (serializer (account2 'serializer))
        (id1 (account1 'id))
        (id2 (account2 'id)))
    (if (< id1 id2)
        ((serializer2 (serializer1 exchange))
          account1
          account2)
        ((serializer1 (serializer2 exchange))
          account1
          account2))))


(define (make-account-and-serializer balance id)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (let ((balance-serializer (make-serializer)))
    (define (dispatch m)
      (cond ((eq? m 'withdraw) widthdraw)
            ((eq? m 'deposit) deposit)
            ((eq? m 'balance) balance-serializer)
            ((eq? m 'serializer) balance-serializer)
            ((eq? m 'id) id)
            (else (error "Unknow request -- MAKE_ACCOUNT"
                         m))))
    dispatch))