HAL Id: inria-00407778 https://inria.hal.science/inria-00407778 Submitted on 28 Jul 2009 HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisci