[1] Univ Brescia, Dipartimento Ingn Informaz, I-25123 Brescia, Italy
[2] Univ Roma La Sapienza, Ist Nazl Fis Nucl, Dipartimento Fis, I-00185 Rome, Italy
[3] Northumbria Univ, Dept Math & Informat Sci, Newcastle Upon Tyne NE2 1XE, Tyne & Wear, England