Academy of Economic Studies, Dept. of Cybernetics Piata, Romana, no.6, Sector 1, Bucharest, Romania;Institute for Computers and Informatics, Dept. BNP, Av. Miciurin, no.8-10, Sector 1, Bucharest, Romania;Institute for Computers and Informatics, Dept. BNP, Av. Miciurin, no.8-10, Sector 1, Bucharest, Romania