1. Dipartimento di Matematica e Informatica, UniversitÀ degli Studi di Palermo, Via Archirafi 34, 90123 Palermo, Italy
2. Bolyai Institute, University of Szeged, Aradi vértanúk tere 1, H-6720 Szeged, Hungary
3. HUN-REN-ELTE Geometric and Algebraic Combinatorics Research Group, Pázmány Péter sétány 1/C, H-1117 Budapest, Hungary.