Authors
Francis Bonahon
Current address:
Department of Mathematics, University of Southern California, 3620 S. Vermont Ave., KAP 104, Los Angeles, CA 90089-2532
Jean-Pierre Otal
Current address:
Institut de Mathématique de Toulouse, Universit&3acute; Paul Sabatier, 118, Route de Narbonne, 31062 Toulouse, France