A combination of the two methods (referred to under Paragraph CA-4.3.1) is not permitted. Any exceptions to this rule will require the prior written approval of the Central Bank. It is expected that such approval will only be given in cases where a bank clearly demonstrates to the Central Bank, the difficulty in applying, to a definite Category of trading instruments, the method otherwise chosen by the bank as the normal method. It is further expected that the Central Bank may, in future years, consider recognising the duration method as the approved method, and the use of the maturity method may be discontinued.