0 Humboldt University Institute of Informatics, Warsaw University University of Bialystok, Department of Mathematics Brandenburg University at Cottbus Bialystok University of Technology Institute of Mathematics, The University of Warsaw IPI PAN and University of Podlasie Fraunhofer FIRST and Humboldt University Warsaw UIniversity Chair of Computer Science, University of Rzeszow, Rzeszow, Poland Warsaw University Institute of Mathematics, The University of Warsaw TU Chemnitz University of Rostock 1 Louchka Popova-Zeugmann Humboldt University, Institut of Computer Science , chair