Agriculture - Common agricultural policy - Structural reform - Improvement in the efficiency of structures - Aid for extensification of production of beef and veal - Conditions for granting - Reduction in output - Calculation - Account taken of drops in output during the intermediate period between the end of the reference period and the beginning of the period of the undertaking - Not permissible (Council Regulation No 797/85, Art. 1b(3)(c), as amended by Regulations Nos 1760/87 and 1094/88; Commission Regulation No 4115/88, Art. 4(1) and (2))