Martin Gebser Torsten Schaub Hans Tompits Stefan Woltran Alternative Characterizations for Program Equivalence under Answer-Set Semantics Based on Unfounded Sets. 24-41 2008 conf/foiks/2008 FoIKS http://dx.doi.org/10.1007/978-3-540-77684-0_5 db/conf/foiks/foiks2008.html#GebserSTW08