SICP問題3.42

withdrawとdepositの呼び出しの度に新しい直列化された手続きを作り出して返すmake-account(教科書で定義)を

(define (make-account balance)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (let ((protected (make-serializer)))
    (define (dispatch m)
      (cond ((eq? m 'withdraw) (protected withdraw))
            ((eq? m 'deposit) (protected deposit))
            ((eq? m 'balance) balance)
            (else (error "Unknown request -- MAKE-ACCOUNT"
                         m))))
    dispatch))

withdrawとdepositの呼び出しの際は定義済の直列化された手続きを返すmake-account(教科書で定義)

(define (make-account balance)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (let ((protected (make-serializer)))
    (let ((protected-withdraw (protected withdraw))
          (protected-deposit (protected deposit)))
      (define (dispatch m)
        (cond ((eq? m 'withdraw) protected-withdraw)
              ((eq? m 'deposit) protected-deposit)
              ((eq? m 'balance) balance)
              (else (error "Unknown request -- MAKE-ACCOUNT"
                           m))))
      dispatch)))

に変更した場合、変更は安全で並列性に違いがあるか?という問題。
安全な変更であり、並列性に違いはない。