Konstantin Korovin

New directions in instantiation-based theorem proving

We consider instantiation-based theorem proving whereby instances of clauses are generated by certain inferences, and where inconsistency is detected by proposition tests. We give a model construction proof of completeness by which restrictive inference systems as well as admissible simplification techniques can be justified. Another contribution of the paper are inference systems that allow one to also employ decision procedures for first-order fragments more complex than propositional logic. The decision provides for an approximate consistency test, and the instance generation inference system is a means of successively refining the approximation.

iProver --- An Instantiation-Based Theorem Prover for First-Order Logic (System Description)

iProver is an instantiation-based theorem prover which is based on Inst-Gen calculus, complete for first-order logic. One of the distinctive features of iProver is a modular combination of instantiation and propositional reasoning. In particular, any state-of-the art SAT solver can be integrated into our framework. iProver incorporates state-of-the-art implementation techniques such as indexing, redundancy elimination, semantic selection and saturation algorithms. Redundancy elimination implemented in iProver include: dismatching constraints, blocking non-proper instantiations and propositional-based simplifications. In addition to instantiation, iProver implements ordered resolution calculus and a combination of instantiation and ordered resolution. In this paper we discuss the design of iProver and related implementation issues.

Integrating linear arithmetic into superposition calculus

We present a method of integrating linear rational arithmetic into superposition calculus for first-order logic. One of our main results is completeness of the resulting calculus under some finiteness assumptions.

Orienting rewrite rules with the Knuth--Bendix order

We consider two decision problems related to the Knuth-Bendix order (KBO). The first problem is orientability: given a system of rewrite rules R, does there exist an instance of KBO which orients every ground instance of every rewrite rule in R. The second problem is whether a given instance of KBO orients every ground instance of a given rewrite rule. This problem can also be reformulated as the problem of solving a single ordering constraint for the KBO. We prove that both problems can be solved in the time polynomial in the size of the input. The polynomial-time algorithm for orientability builds upon an algorithm for solving systems of homogeneous linear inequalities over integers. We show that the orientability problem is P-complete. The polynomial-time algorithm for solving a single ordering constraint does not need to solve systems of linear inequalities and can be run in time O(n2). Also we show that if a system is orientable using a real-valued instance of KBO, then it is also orientable using an integer-valued instance of KBO. Therefore, all our results hold both for the integer-valued and the real-valued KBO.

Theory instantiation

In this paper we present a method of integrating theory reasoning into the instantiation framework. This integration is done in the black-box style, which allows us to integrate different theories in a uniform way. We prove completeness of the resulting calculus, provided that the theory reasoner is answer-complete and complete for reasoning with ground clauses. One of the distinctive features of our approach is that it allows us to employ off-the-shelf satisfiability solvers for ground clauses modulo theories, as a part of general first-order reasoning. As an application of this approach, we show how it is possible to combine the instantiation calculus with other calculi, such as ordered resolution and paramodulation.

Inst-Gen – A Modular Approach to Instantiation-Based Automated Reasoning

Inst-Gen is an instantiation-based reasoning method for first-order logic introduced in [18]. One of the distinctive features of Inst-Gen is a modular combination of first-order reasoning with efficient ground reasoning. Thus, Inst-Gen provides a framework for utilising efficient off-the-shelf propositional SAT and SMT solvers as part of general first-order reasoning. In this paper we present a unified view on the developments of the Inst-Gen method: (i) completeness proofs; (ii) abstract and concrete criteria for redundancy elimination, including dismatching constraints and global subsumption; (iii) implementation details and evaluation.

Integrating Equational Reasoning into Instantiation-Based Theorem Proving

In this paper we present a method for integrating equational reasoning into instantiation-based theorem proving. The method employs a satisfiability solver for ground equational clauses together with an instance generation process based on an ordered paramodulation type calculus for literals. The completeness of the procedure is proved using the the model generation technique, which allows us to justify redundancy elimination based on appropriate orderings.

Instantiation-Based Automated Reasoning: From Theory to Practice

Instantiation-based automated reasoning aims at combining the efficiency of propositional SAT and SMT technologies with the expressiveness of first-order logic. Propositional SAT and SMT solvers are probably the most successful reasoners applied to real-world problems, due to extremely efficient propositional methods and optimized implementations. However, the expressiveness of first-order logic is essential in many applications ranging from formal verification of software and hardware to knowledge representation and querying. Therefore, there is a growing demand to integrate efficient propositional and more generally ground reasoning modulo theories into first-order reasoning.

A decision procedure for the existential theory of term algebras with the Knuth-Bendix ordering

The authors show the decidability of the existential theory of term algebras with any Knuth-Bendix ordering. They achieve this by giving a procedure for solving Knuth-Bendix ordering constraints. As for complexity, NP-hardness of the set of satisfiable quantifier-free formulas can be shown in the same way as by R. Nieuwenhuis (1993). The algorithm presented does not give an NP upper bound; we point out parts of our algorithm that may cause nonpolynomial behavior.

iProver-Eq: an instantiation-based theorem prover with equality

iProver-Eq is an implementation of an instantiation-based calculus Inst-Gen-Eq which is complete for first-order logic with equality. iProver-Eq extends the iProver system with superposition-based equational reasoning and maintains the distinctive features of the Inst-Gen method. In particular, first-order reasoning is combined with efficient ground satisfiability checking where the latter is delegated in a modular way to any state-of-the-art SMT solver. The first-order reasoning employs a saturation algorithm making use of redundancy elimination in form of blocking and simplification inferences. We describe the equational reasoning as it is implemented in iProver-Eq, the main challenges and techniques that are essential for efficiency.