HAL Id: hal-00691958 https://inria.hal.science/hal-00691958v2 Submitted on 6 Jun 2012 (v2), last revised 25 Jul 2012 (v3) 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 cent