1. Department of Mathematics, Indiana University Bloomington, 831 E. Third Street, Bloomington, IN 47405, USA
2. Center for Integrative Genomics, University of Lausanne,1015 Lausanne, Switzerland
3. Swiss Institute of Bioinformatics, 1015 Lausanne, Switzerland
4. Centre for Mathematical Biology, and Department of Mathematical Sciences, University of Bath, North Rd, Bath BA2 7AY, England