Thomas Eiter Georg Gottlob Heikki Mannila Expressive Power and Complexity of Disjunctive Datalog under the Stable Model Semantics. 83-103 1994 conf/iski/1994 IS/KI db/conf/iski/iski94.html#EiterGM94 http://dx.doi.org/10.1007/3-540-57802-1_5