https://cel.archives-ouvertes.fr/cel-01947220 Contributor : François SchwarzentruberConnect in order to contact the contributor Submitted on : Thursday, December 6, 2018 - 3:29:50 PM Last modification on : Saturday, January 19, 2019 - 1:18:26 AM Long-term archiving on: : Thursday, March 7, 2019 - 2:15:31 PM