In the light of the system of value added tax resulting from Council Directive 2006/112/EC, is a provision of national law permissible which introduces derogations from the general rules on that tax, in particular with regard to the persons required to calculate and collect the tax, by establishing the concept of paying agent, that is to say, a person who is required, on behalf of the taxable person, to calculate the amount of tax, collect it from the taxable person, and pay it to the tax authority in good time?