Institute for Theoretical Biology, Humboldt University and Charité, Berlin, Germany. bernard@math.univ-lyon1.fr|Institute for Theoretical Biology, Humboldt University and Charité, Berlin, Germany. bernard@math.univ-lyon1.fr|Institute for Theoretical Biology, Humboldt University and Charité, Berlin, Germany. bernard@math.univ-lyon1.fr|Institute for Theoretical Biology, Humboldt University and Charité, Berlin, Germany. bernard@math.univ-lyon1.fr