1. Department of Informatics, Systems, and Production University of Rome Tor Vergata Via del Politecnico 1, 00133 Rome, Italy. E-mail: senni@disp.uniroma2.it; pettorossi@disp.uniroma2.it
2. IASI-CNR Viale Manzoni 30, 00185 Rome, Italy. E-mail: maurizio.proietti@iasi.cnr.it