1. Mumps Technologies, ENS Lyon, 46 Allée d’Italie, F-69007 Lyon, France.
2. CNRS, IRIT, 2 Rue Charles Camichel, F-31071 Toulouse, France.
3. Department of Mathematics, The University of Manchester, Manchester, M13 9PL, UK.
4. Sorbonne Université, CNRS, LIP6, Paris, F-75005, France.