1. Dipartimento di Matematica, University of Parma, Italy. E-mail: alessandro.dalpalu@unipr.it
2. Dipartimento di Matematica e Informatica, University of Udine, Italy. E-mail: dovier@dimi.uniud.it
3. Dept. Computer Science, New Mexico State University, Mexico. E-mail: epontell@cs.nmsu.edu
4. Dipartimento di Matematica, University of Parma, Italy. E-mail: gianfranco.rossi@unipr.it