CSIRO Mathematical and Information Sciences, GPO Box 664, Canberra ACT 2601, Australia. Bob.Anderssen@csiro.au|CSIRO Mathematical and Information Sciences, GPO Box 664, Canberra ACT 2601, Australia. Bob.Anderssen@csiro.au|CSIRO Mathematical and Information Sciences, GPO Box 664, Canberra ACT 2601, Australia. Bob.Anderssen@csiro.au|CSIRO Mathematical and Information Sciences, GPO Box 664, Canberra ACT 2601, Australia. Bob.Anderssen@csiro.au