1. Institut Montpelliérain Alexander Grothendieck, UMR 5149 CNRS, Université de Montpellier, Case Courrier 051, Place Eugène Bataillon, 34095 Montpellier Cedex, France
2. Faculty of Mathematics and Computer Science, University of Lodz, ul. Banacha 22, 90-230 Łȯdź, Poland