1. IEC, Department of Information Engineering, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
2. Institute of Mathematics and Computer Science, Academy of Sciences of Moldova, Academiei 5, Chişinău MD-2028, Moldova
3. Research Group on Mathematical Linguistics, Rovira i Virgili University, Av. Catalunya, 35, Tarragona 43002, Spain
4. LACL, Département Informatique, Université Paris Est, 61 av. Général de Gaulle, 94010 Créteil, France