1. Dipartimento di Informatica, Sistemistica e Comunicazione, Università di Milano-Bicocca degli Studi, Viale Sarca 336, 20126 Milano, Italy. artiom@math.md
2. Research Group on Mathematical Linguistics, Rovira i Virgili University, Av. Catalunya 35, 43002 Tarragona, Spain. alexander.krassovitskiy@estudiants.urv.cat
3. Institute of Mathematics and Computer Science, Academy of Sciences of Moldova, Academiei 5, Chişinău MD-2028, Moldova. rogozhin@math.md
4. Département Informatique, Université Paris Est, 61 av. Général de Gaulle, 94010 Créteil, France. verlan@univ-paris12.fr