1. Department of Computer Science, University of Pisa, Pisa, PI, Italy
2. Department of Informatics and Telematics and School of Electrical and Computer Engineering, Harokopio University of Athens and National Technical University of Athens, Athens, Attika, Greece
3. Department of Computer Science and Institute of Information Science and Technologies, University of Pisa and National Research Council of Italy, Pisa, PI, Italy
4. Institute of Information Science and Technologies, National Research Council of Italy, Pisa, PI, Italy
5. Institute of Informatics and Telematics, National Research Council of Italy, Pisa, PI, Italy