[1] Computer and Automation Research Institute, Hungarian Academy of Sciences, Kende u. 13-17, H-1111 Budapest, Hungary
[2] Research Group on Mathematical Linguistics, Rovira i Virgili University, Pça. Imperial Tàrraco 1, 43005 Tarragona, Spain
[3] University of Bucharest, Faculty of Mathematics, Str. Academiei 14, 70109 Bucharest, Romania