1. B. Korber is in the Theoretical Division (T-10), Los Alamos National Laboratory, Los Alamos, NM 87545, USA, and is a visiting faculty member at the Santa Fe Institute, Santa Fe, NM 87501, USA. J. Theiler is in the Space and Remote Sensing Sciences Group (NIS-2), Los Alamos National Laboratory, Los Alamos, NM 87545, USA. S. Wolinsky is in the Department of Medicine, Northwestern University Medical School, Chicago, IL 60611, USA.