1. Department of Computer Science, Upper Mountjoy Campus, Durham University, Durham DH1 3LE, UK.
2. Department of Mathematics, University of Manchester, Manchester M13 9PL, UK.
3. Livermore Software Technology, ANSYS Company, Livermore, CA 94551 USA.
4. Sorbonne Université, CNRS, LIP6, Paris 75005, France.