1. Department of Mathematics and Statistics, Dalhousie University, Halifax, Nova Scotia, Canada
2. Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada
3. Department of Mathematics, University of Auckland, Private Bag, Auckland, New Zealand
4. McGill Centre for Bioinformatics, McGill University, 3775 University Street, Duff Medical Building, Montreal, Quebec, Canada