Revisiting Unrestricted Rebut and Preferences in Structured Argumentation.

In structured argumentation frameworks such as ASPIC+, rebuts are only allowed in conclusions produced by defeasible rules. This has been criticized as counter-intuitive especially in dialectical contexts. In this paper we show that ASPIC-, a system allowing for unrestricted rebuts, suffers from contamination problems. We remedy this shortcoming by generalizing the attack rule of unrestricted rebut. Our resulting system satisfies the usual rationality postulates for prioritized rule bases.