1. DEPARTMENT OF MATHEMATICS; UNIVERSITY OF WEST BOHEMIA; P.O. BOX 314 306 14 PILSEN CZECH REPUBLIC
2. INSTITUTE FOR THEORETICAL COMPUTER SCIENCE (ITI); CHARLES UNIVERSITY; P.O. BOX 314 306 14 PILSEN CZECH REPUBLIC
3. SCHOOL OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE; THE UNIVERSITY OF NEWCASTLE; AUSTRALIA