1. Institute of Theoretical Physics, University of Wroclaw, pl. M. Borna 9, 50-204 Wroclaw, Poland
2. Rudjer Bošković Institute, Bijenička c.54, HR-10002 Zagreb, Croatia
3. Dipartimento di Matematica “Giuseppe Peano”, Universita degli Studi di Torino, Via Carlo Alberto, 10–10123 Torino, Italy