Monadic second-order incorrectness logic for GP 2

Program logics typically reason about an over-approximation of program behaviour to prove the absence of bugs. Recently, program logics have been proposed that instead prove the presence of bugs by means of under-approximate reasoning, which has the promise of better scalability. In this paper, we present an under-approximate program logic for GP 2, a rule-based programming language for manipulating graphs. We define the proof rules of this program logic extensionally, i.e. independently of fixed assertion languages, then instantiate them with a morphism-based assertion language able to specify monadic second-order properties on graphs (e.g. path properties). We show how these proof rules can be used to reason deductively about the presence of forbidden graph structure or failing executions. Finally, we prove our ‘incorrectness logic’ to be sound, and our extensional proof rules to be relatively complete.