Victor W. Marek Inna Pivkina Miroslaw Truszczynski Approximating Answer Sets of Unitary Lifschitz-Woo Programs. 66-78 2005 conf/lpnmr/2005 LPNMR http://dx.doi.org/10.1007/11546207_6 db/conf/lpnmr/lpnmr2005.html#MarekPT05