TAX PROVISIONS - HARMONIZATION OF LAWS - TURNOVER TAXES - COMMON SYSTEM OF VALUE-ADDED TAX - BASIS OF THE CHARGE TO TAX - NATIONAL DEROGATING MEASURES - AMENDMENT OF A MEASURE IN FORCE - OBLIGATION TO NOTIFY THE COMMISSION - FAILURE TO NOTIFY - AMENDMENT MAY NOT BE RELIED UPON AS AGAINST INDIVIDUALS ( COUNCIL DIRECTIVE NO 77/388 , ART . 11 A 1 . ( A ) AND ART . 27 ( 1 ), ( 2 ) AND ( 5 ))