From Disjunctive to Normal Logic Programs via Unfolding and Shifting

We show that every propositional disjunctive logic program under the answer set semantics can be equivalently transformed into a normal one via unfolding and shifting. More precisely, after iteratively applying the unfolding operator for some rules in a disjunctive program, its shifted program, which is a normal program, must have the same answer sets as the original disjunctive program.