[1] Univ Ljubljana, Fac Math & Phys, Ljubljana, Slovenia
[2] Univ Maribor, Fac Nat Sci & Math, SLO-2000 Maribor, Slovenia
[3] Nankai Univ, Ctr Combinator, Tianjin 300071, Peoples R China
[4] Nankai Univ, LPMC, Tianjin 300071, Peoples R China
[5] Univ Melbourne, Sch Math & Stat, Parkville, Vic 3010, Australia