1 . Agriculture - Common organization of the markets - Discrimination between producers or consumers - Co-responsibility levy in the cereals sector - Exemption for cereals used on the producer' s farm after processing - Exemption subject to cereals being processed on the farm - Unlawful ( EEC Treaty, second subparagraph of Art . 40 ( 3 ); Commission Regulation No 2040/86, second subparagraph of Art . 1 ( 2 ), as amended by Regulation No 2572/86 ) 2 . References for a preliminary ruling - Assessment of validity - Declaration that a regulation is invalid - Effects - Application by analogy of the second paragraph of Article 174 of the Treaty - Invalidity resulting from discrimination - Provisional maintenance in force of the contested scheme on the basis of non-discriminatory rules ( EEC Treaty, Art . 174 ( 2 ) and Art . 177 )