PETER HALL, SPIRIDON PENEV, GE RARD KERKYACHARIAN and DOMINIQUE PICARD Centre for Mathematics and its Applications, Australian National University, Canberra, ACT 0200, Australia School of Mathematics, University of NSW, Sydney, NSW 2052, Australia Faculte Mathematiques et Informatiques, Universite de Picardie, 33 rue Saint-Leu, 80039 Amiens Cedex 01, France DeÂpartement de Mathematiques, Uni...