1. Mathematical Sciences Institute, The Australian National University, J. Dedman Bldg 27, Union Lane, Canberra ACT 2601, Australia
2. Faculty of Mathematics and Computer Science, Nicolaus Copernicus University in Toruń, ul. Chopina 12/18, 87-100 Toruń, Poland
3. Department of Geoinformatics, Faculty of Science, Palacky University Olomouc, 17. Listopadu 50, Olomouc 771 46, Czech Republic