(9)Full harmonisation is necessary in order to ensure that all consumers in the Community enjoy a high and equivalent level of protection of their interests and to create a genuine internal market. Member States should therefore not be allowed to maintain or introduce national provisions other than those laid down in this Directive. However, such restriction should only apply where there are provisions harmonised in this Directive. Where no such harmonised provisions exist, Member States should remain free to maintain or introduce national legislation. Accordingly, Member States may, for instance, maintain or introduce national provisions on joint and several liability of the seller or the service provider and the creditor. Another example of this possibility for Member States could be the maintenance or introduction of national provisions on the cancellation of a contract for the sale of goods or supply of services if the consumer exercises his right of withdrawal from the credit agreement. In this respect Member States, in the case of open-end credit agreements, should be allowed to fix a minimum period needing to elapse between the time when the creditor asks for reimbursement and the day on which the credit has to be reimbursed.