[1] Department of Computer Science, University of Crete, GR 71409 Heraklion, Crete, Greece
[2] Institute of Computer Science, Foundation for Research and Technology-Hellas (FORTH), Science and Technology Park of Crete, Vassilika Vouton, P.O.Box 1385, GR 71110 Heraklion, Crete, Greece
[3] Università Roma Tre, Dip. Informatica e Automazione, Via della Vasca Navale, 79, 00146 Rome, Italy