1. Institute of Informatics and Telematics, National Research Council, Via G. Moruzzi 1, 56124 Pisa, Italy
2. Department of Information Engineering, University of Pisa, L. Lazzarino 1, 56122 Pisa, Italy
3. Department of Computer Science & Applied Statistics, University of New Brunswick, Saint John, NB, Canada E2L 4L5