IRISA-INRIA, Campus de Beaulieu, 35402 Rennes cedex, France. stan@irisa.fr|IRISA-INRIA, Campus de Beaulieu, 35402 Rennes cedex, France. stan@irisa.fr|IRISA-INRIA, Campus de Beaulieu, 35402 Rennes cedex, France. stan@irisa.fr|IRISA-INRIA, Campus de Beaulieu, 35402 Rennes cedex, France. stan@irisa.fr|IRISA-INRIA, Campus de Beaulieu, 35402 Rennes cedex, France. stan@irisa.fr