1. a The Rugjer Bošković Institute , P.O.B. 1016, HR-41001, Zagreb , The Republic of Croatia
2. b Dipartimento di Fisica and INFM-CISM , Viale delle Scienze , 43100, Parma , (Italy)
3. c PROGEGA s.n.c. Via Mario Mencatelli , 14,00128, Rome , (Italy)
4. d Thinking Machines Corporation , Cambridge , MA , 02142-1214 , (USA)
5. e College of Computer Science, Northeastern University , Boston , MA , 02115 , (USA)