1. Department of Mathematics, University of Alberta, Edmonton, Alberta Canada T6G 2G1, Canada
2. National Resource Information Center, Canberra A.C.T. 2600, Australia
3. Biol. Research Center, Branišovska 31, 370 05 České Budejovice, Czech Republic
4. Department of Mathematics, Hokkaido Tokai Univ., Sapporo, 005, Japan