++++ Tax provisions - Harmonization of legislation - Turnover taxes - Common system of value-added tax - Taxable base - National derogating measures - Policy of combating tax evasion or avoidance - Measure applicable to a taxable person acting without any intention of obtaining a tax advantage - Measure designed to combat tax avoidance - Measure applicable only to certain taxable persons whose sales methods lead to the avoidance of tax - Whether permissible - Conditions ( Council Directive 77/388/EEC, Arts 11A.1 ( a ) and 27 ( 1 ) )