1. Parallel Systems, Department of Computing Science, Carl von Ossietzky Universität Oldenburg, D-26111 Oldenburg, Germany. eike.best@informatik.uni-oldenburg.de
2. INRIA, Centre Rennes - Bretagne Atlantique, Campus de Beaulieu, F-35042 Rennes Cedex, France. Philippe.Darondeau@inria.fr