1. Laboratoire SRSI, URA CNRS 1662, Université P. et M. Curie (Paris VI), B. P. 52, 4 Place Jussieu, F‐75231 Paris Cedex 05 (France)
2. Department of Applied Mathematics, Australian National University, PO Box 4, ACT Canberra, 0200 (Australia)
3. Centre de Génétique Moléculiere‐CNRS, F‐91190 Gif‐sur‐Yvette Cedex (France)