1. Department of Informatics, Systems and Communications, University of Milano-Bicocca, 20126 Milano, Italy. cattang@live.it; cattang@fislab.disco.unimib.it
2. Department of Mathematics and Informatics, University of Calabria, 87036 Arcavacata di Rende (CS), Italy. chiaselotti@unical.it; gentile@mat.unical.it; oliverio@unical.it