1. Mathematisches Institut, Ludwig-Maximilans Universität München, Germany; and Department of Mathematics, California Institute of Technology, Pasadena, Calif., U.S.A.
2. IRMAR, Université de Rennes, France; and INRIA, IPSO Project, Campus de Beaulieu, Rennes, France
3. Department of Mathematics, Statistics, and Computer Science, University of Illinois, Chicago, Il., U.S.A.