1Theoretical Division (MS B284); Los Alamos National Laboratory; Los Alamos; NM 87545; U.S.A.
2Department of Biological Statistics and Computational Biology; Cornell University; 432 Warren Hall;
Ithaca; NY 14853; U.S.A.
3Department of Mathematics and Statistics; Arizona State University; P.O. Box 871804; Tempe;
AZ 85287-1804; U.S.A.
4Department of Mathematics; The University of British Columbia; Vancouver; B.C.; Canada V6T 1Z2
5Instituto Mexicano del Seguro Social (IMSS); Colima; Col. 28010; Mexico