We review the notion of perfect recall in the literature on interpretedsystems, game theory, and epistemic logic. In the context of Epistemic TemporalLogic (ETL), we give a (to our knowledge) novel frame condition for perfectrecall, which is local and can straightforwardly be translated to a definingformula in a language that only has next-step temporal operators. This framecondition also gives rise to a complete axiomatization for S5 ETL frames withperfect recall. We then consider how to extend and consolidate the notion ofperfect recall in sub-S5 settings, where the various notions discussed are nolonger equivalent.
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