Jos de Bruijn Stijn Heymans Complexity of the Stable Model Semantics for Queries on Incomplete Databases. 101-114 2009 LPNMR http://dx.doi.org/10.1007/978-3-642-04238-6_11 conf/lpnmr/2009 db/conf/lpnmr/lpnmr2009.html#BruijnH09