1. Department of Computer Science University of Helsinki Gustaf Hällströmin Katu 2b 00014 Helsinki Finland
2. Department of Electrical & Information Engineering (DEI) Polytechnic University of Bari Via E. Orabona 4 70125 Bari Italy
3. Department of Mathematics and Computer Science University of Perugia Via Vanvitelli 1 06123 Perugia Italy