++++ Tax provisions - Harmonization of laws - Turnover tax - Common system of value-added tax - Exemptions provided for by the Sixth Directive - Exemption for transactions consisting of the negotiation of credit - Possibility for individuals to rely upon the relevant provision in the absence of implementation of the directive - Conditions - Period concerned ( Council Directives 77/388/EEC, Art . 13 B ( d ) ( 1 ), and 78/583/EEC, Art . 1 )