1. Department of Digital Systems, University of Piraeus, Athens, Greece
2. Department of Informatics & Telecommunications, University of Athens, Athens, Greece
3. Software and Knowledge Engineering Laboratory, Institute of Informatics and Telecommunications, National Centre for Scientific Research (N.C.S.R.) “Demokritos”, GR-153 10, P. O. Box 60228, Aghia Paraskevi, Athens, Greece