Concurrence for well-formed CAFs: Naive Semantics

In the area of claim-based reasoning in abstract argumentation, a claim-based semantics is said to be concurrent in a given framework if all its variants yield the same extensions. In this note, we show that the concurrence problem with respect to naive semantics is coNP-hard for well-formed CAFs. This solves a problem that has been left open in Dvor\'ak et al. (2021).