LIPIcs.FSTTCS.2024.9.pdf
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When reasoning about games, one is often interested in verifying more intricate strategic properties than the mere existence of a winning strategy for a given coalition. Several languages, among which the very expressive Strategy Logic (SL), have been proposed that explicitly quantify over strategies in order to express and verify such properties. However, quantifying over strategies poses serious issues: not only does this lead to a non-elementary model-checking problem, but the classic Tarskian semantics is not fully adequate, both from a conceptual and practical viewpoint, since it does not guarantee the realisability of the strategies involved. In this paper, we follow a different approach and introduce Plan Logic (PL), a logic that takes plans, i.e., sequences of actions, as first-class citizens instead of strategies. Since plans are much simpler objects than strategies, it becomes easier to enforce realisability. In this setting, we can recover strategic reasoning by means of a compositional hyperteams semantics, inspired by the well-known team semantics. We show that the Conjunctive-Goal and Disjunctive-Goal fragments of SL are captured by PL, with an effective polynomial translation. This result relies on the definition of a suitable game-theoretic semantics for the two fragments. We also investigate the model-checking problem for PL. For the full prenex fragment, the problem is shown to be fixed-parameter-tractable: it is polynomial in the size of the model, when the formula is fixed, and 2-ExpTimeC in the size of the formula. For the Conjunctive-Goal and Disjunctive-Goal fragments of PL this result can be improved to match the optimal polynomial complexity in the size of the model, regardless of the size of the formula.
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