Accrual of arguments in defeasible argumentation

In this paper we address an often overlooked problem in defeasible argumentation: how do we deal with arguments that are on their own defeated, but together remain undefeated? Pollock (1991) finds this accrual of arguments a natural supposition, but then surprisingly denies its existence. We think that arguments do accrue. To handle the accrual of arguments, we introduce compound defeat of arguments. We call the defeat of arguments compound, if groups of arguments can be defeated by other groups of arguments. The formalism presented in this paper is based on this notion of compound defeat. It adequately handles the accrual of arguments.