1. Department of Statistics, University of British Columbia, Vancouver, BC Canada V6T 1Z4
2. Department of Statistics and Actuarial Science, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
3. National Institute of Allergy and Infectious Diseases, National Institutes of Health, Maryland 20892, U.S.A.
4. Department of Statistics and Applied Probability, National University of Singapore, 117546 Singapore