Characterizing contrasimilarity through games, modal logic, and complexity

Abstract

We present the first game characterization of contrasimilarity, the weakest form of bisimilarity. It corresponds to an elegant modal characterization of nested trees of impossible future behavior. The game is exponential but finite for finite-state systems and can thus be used for contrasimulation equivalence checking, of which no tool has been capable to date. By reduction from weak trace equivalence, we establish that contrasimilarity is PSPACE-complete. A machine-checked Isabelle/HOL formalization backs our work and enables further use of contrasimilarity in verification contexts.