European Union law does not preclude national legislation prohibiting the retroactive application of a special tax scheme granting an exemption to small enterprises — adopted pursuant to the provisions of Title XII, Chapter 1, Section 2, of Council Directive 2006/112/EC — to a taxable person who fulfilled all the material conditions but did not declare the commencement of his activities to the tax authorities in due course, and did not opt for the application of that scheme.