David Billington Grigoris Antoniou Guido Governatori Michael J. Maher Revising Nonmonotonic Theories: The Case of Defeasible Logic. 101-112 http://link.springer.de/link/service/series/0558/bibs/1701/17010101.htm 1999 conf/ki/1999 KI db/conf/ki/ki99.html#BillingtonAGM99