 Bills of exchange also fall within the definition of book debts, but a bank balance does not.Under (for example) English law, any general assignment, either absolute or by way of security, of book debts is void unless registered under the Bills of Sale Act 1878.A trustee would not be able to attack an assignment under this section which relates to debts due from specified debtors or debts becoming due under specified contracts or where the debts were assigned as part of a "bona fide" transfer of a business or the assignment is for the benefit of creditors generally.