Guido Governatori Antonino Rotolo Changing Legal Systems: Abrogation and Annulment Part I: Revision of Defeasible Theories. 3-18 2008 DEON http://dx.doi.org/10.1007/978-3-540-70525-3_2 conf/deon/2008 db/conf/deon/deon2008.html#GovernatoriR08