http://www.polarfle.com/exercice/exodebnegation.htm
http://www.lepointdufle.net/p/negation.htm
http://www.polarfle.com/exercice/exonegdeelem.htm
http://www.francaisfacile.com/cgi2/myexam/liaison.php?liaison=_negation_&id=33354&auteur=sallma