Carsten Lutz Dirk Walther 0002 Frank Wolter Conservative Extensions in Expressive Description Logics. 453-458 2007 conf/ijcai/2007 IJCAI http://dli.iiit.ac.in/ijcai/IJCAI-2007/PDF/IJCAI07-071.pdf db/conf/ijcai/ijcai2007.html#LutzWW07