1. Department of Systems Biology and NSF-Simons Center for Mathematical and Statistical Analysis of Biology, Harvard University, Cambridge, MA 02138 USA.
2. Department of Pure Mathematics, University of Waterloo, Waterloo N2L 3G1, ON, Canada.
3. Scuola Internazionale Superiore di Studi Avanzati, Trieste 34136, Italy, and Lucian Blaga University of Sibiu, Sibiu 550012, Romania.
4. NSF-Simons Center for Mathematical and Statistical Analysis of Biology, Harvard University, Cambridge, MA 02138 USA.