HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entic 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 pluridisciplinaire HAL, est
destinee au dep^ot et a la diusion de documents
scientiques de niveau recherche, publies ou non,
emanant des etablissements d'enseignement et de
recherche francais ou etrangers, des laboratoires
publics ou prives.