Joohyung Lee Yunsong Meng On Reductive Semantics of Aggregates in Answer Set Programming. 182-195 2009 LPNMR http://dx.doi.org/10.1007/978-3-642-04238-6_17 conf/lpnmr/2009 db/conf/lpnmr/lpnmr2009.html#LeeM09