1. Toulouse Institute of Computer Science Research, CNRS-INPT-UT3, Toulouse University , 31062 Toulouse, France
2. Toulouse Institute of Computer Science Research, CNRS-INPT-UT3, Toulouse University , 31062 Toulouse, France and Faculty of Arts and Sciences, Istanbul Aydın University, 34295 Istanbul, Turkey
3. Faculty of Mathematics and Informatics, Sofia University St . Kliment Ohridski, 1164 Sofia, Bulgaria