(111)Contracting authorities should, in the individual contracts themselves, have the possibility to provide for modifications to a contract by way of review or option clauses, but such clauses should not give them unlimited discretion. This Directive should therefore set out to what extent modifications may be provided for in the initial contract. It should consequently be clarified that sufficiently clearly drafted review or option clauses may for instance provide for price indexations or ensure that, for example, communications equipment to be delivered over a given period continues to be suitable, also in the case of changing communications protocols or other technological changes. It should also be possible under sufficiently clear clauses to provide for adaptations of the contract which are rendered necessary by technical difficulties which have appeared during operation or maintenance. It should also be recalled that contracts could, for instance, include both ordinary maintenance as well as provide for extraordinary maintenance interventions that might become necessary in order to ensure continuation of a public service.