Thomas Eiter Gerhard Brewka Minh Dao-Tran Michael Fink Giovambattista Ianni Thomas Krennwallner Combining Nonmonotonic Knowledge Bases with External Sources. 18-42 2009 FroCos http://dx.doi.org/10.1007/978-3-642-04222-5_2 conf/frocos/2009 db/conf/frocos/frocos2009.html#EiterBDFIK09