Abstract Building on the small-model constructions for Åqvist’s deontic logics introduced in [24], we present Deo-SMT , an SMT-based reasoner implemented in Z3 for checking validity and generating countermodels. Deo-SMT covers all four of Åqvist’s logics ( E , F , F+(CM) , G ) and provides countermodel visualizations as text, matrices, and directed graphs. Our tool outperforms the existing Isabelle/HOL approach, while providing a lightweight and accessible interface for normative reasoning.

We present a computationally grounded semantics for counterfactual conditionals in which i) the state in a model is decomposed into two elements: a propositional valuation and a causal base in propositional form that represents the causal information available at the state; and ii) the comparative similarity relation between states is computed from the states' two components. We show that, by means of our semantics, we can elegantly formalize the notion of actual cause without recurring to the primitive notion of intervention. Furthermore, we provide a succinct formulation of the model checking problem for a language of counterfactual conditionals in our semantics. We show that this problem is PSPACE-complete and provide a reduction of it into QBF that can be used for automatic verification of causal properties.

We introduce modular calculi for the logics for nonmonotonic reasoning defined by Kraus, Lehmann, and Magidor, featuring a strengthened form of analyticity. Our calculi are used to determine the computational complexity for the logics C, CL, CM, P (and M), and fragments thereof. The calculi are encoded into SMT solvers, yielding an efficient prover with countermodel generation capabilities. Our work encompasses known results and introduces new findings, including co-NP-completeness and a more effective semantics for C.

Input/Output (I/O) logic is a general framework for reasoning about conditional norms and/or causal relations. We streamline Bochman’s causal I/O logics via proof-search-oriented sequent calculi. Our calculi establish a natural syntactic link between the derivability in these logics and in the original I/O logics. As a consequence of our results, we obtain new, simple semantics for all these logics, complexity bounds, embeddings into normal modal logics, and efficient deduction methods. Our work encompasses many scattered results and provides uniform solutions to various unresolved problems.

Abstract In sequent calculi, cut elimination is a property that guarantees that any provable formula can be proven analytically. For example, Gentzen’s classical and intuitionistic calculi LK and LJ enjoy cut elimination. The property is less studied in coinductive extensions of sequent calculi. In this paper, we use coinductive Horn clause theories to show that cut is not eliminable in a coinductive extension of LJ, a system we call CLJ. We derive two further practical results from this study. We show that CoLP by Gupta et al. gives rise to cut-free proofs in CLJ with fixpoint terms, and we formulate and implement a novel method of coinductive theory exploration that provides several heuristics for discovery of cut formulae in CLJ.

We describe a search optimization technique for implementation of relational programming language miniKanren which makes more queries converge. Specifically, we address the problem of conjunction non-commutativity. Our technique is based on a certain divergence criterion that we use to trigger a dynamic reordering of conjuncts. We present a formal semantics of a miniKanren-like language and prove that our optimization does not compromise already converging programs, thus, being a proper improvement. We also present the prototype implementation of the improved search and demonstrate its application for a number of realistic specifications.