Is Article 401 of Council Directive 2006/112/EC to be interpreted as precluding a national tax which applies generally and is proportionate to the price, but which, pursuant to the relevant provisions, is to be levied only at the stage of the sale of goods or services to a consumer, with the result that the final tax burden rests ultimately with the consumer, and which compromises the operation of the common system of value added tax and distorts competition?