[1] Univ Warsaw, Fac Math Informat & Mech, Ul Banacha 2, PL-02097 Warsaw, Poland
[2] Univ Torino, Dipartimento Matemat Giuseppe Peano, Via Carlo Alberto 10, I-10123 Turin, Italy
[3] Univ Oxford, Math Inst, Radcliffe Observ Quarter, Andrew Wiles Bldg,Woodstock Rd, Oxford OX2 6GG, England