Decidable Approximations of Sets of Descendants and Sets of Normal Forms - extended version -

We present here decidable approximations of sets of descendants and sets of normal forms of Term Rewriting Systems, based on specific tree automata techniques. In the context of rewriting logic, a Term Rewriting System is a program, and a normal form is a result of the program. Thus, approximations of sets of descendants and sets of normal forms provide tools for analysing a few properties of programs: we show how to compute a superset of results, to prove the sufficient completeness property, or to find a criterion for proving termination under a specific strategy, the sequential reduction strategy

Unified Analysis of Collapsible and Ordered Pushdown Automata via Term Rewriting

We model collapsible and ordered pushdown systems with term rewriting, by encoding higher-order stacks and multiple stacks into trees. We show a uniform inverse preservation of recognizability result for the resulting class of term rewriting systems, which is obtained by extending the classic saturation-based approach. This result subsumes and unifies similar analyses on collapsible and ordered pushdown systems. Despite the rich literature on inverse preservation of recognizability for term rewrite systems, our result does not seem to follow from any previous study.Comment: in Proc. of FRE

Towards Static Analysis of Functional Programs using Tree Automata Completion

This paper presents the first step of a wider research effort to apply tree automata completion to the static analysis of functional programs. Tree Automata Completion is a family of techniques for computing or approximating the set of terms reachable by a rewriting relation. The completion algorithm we focus on is parameterized by a set E of equations controlling the precision of the approximation and influencing its termination. For completion to be used as a static analysis, the first step is to guarantee its termination. In this work, we thus give a sufficient condition on E and T(F) for completion algorithm to always terminate. In the particular setting of functional programs, this condition can be relaxed into a condition on E and T(C) (terms built on the set of constructors) that is closer to what is done in the field of static analysis, where abstractions are performed on data.Comment: Proceedings of WRLA'14. 201

Rewrite Closure and CF Hedge Automata

We introduce an extension of hedge automata called bidimensional context-free hedge automata. The class of unranked ordered tree languages they recognize is shown to be preserved by rewrite closure with inverse-monadic rules. We also extend the parameterized rewriting rules used for modeling the W3C XQuery Update Facility in previous works, by the possibility to insert a new parent node above a given node. We show that the rewrite closure of hedge automata languages with these extended rewriting systems are context-free hedge languages

Computing with Classical Real Numbers

There are two incompatible Coq libraries that have a theory of the real numbers; the Coq standard library gives an axiomatic treatment of classical real numbers, while the CoRN library from Nijmegen defines constructively valid real numbers. Unfortunately, this means results about one structure cannot easily be used in the other structure. We present a way interfacing these two libraries by showing that their real number structures are isomorphic assuming the classical axioms already present in the standard library reals. This allows us to use O'Connor's decision procedure for solving ground inequalities present in CoRN to solve inequalities about the reals from the Coq standard library, and it allows theorems from the Coq standard library to apply to problem about the CoRN reals

Automata and Equations based Approximations for Reachability Analysis

Invited talkInternational audienceTerm Rewriting Systems (TRSs for short) are a convenient formal model for software systems. This formalism is expressive enough to model in a simple and accurate way many aspects of computation such as: recursivity, non-determinism, parallelism, distribution, communication. On such models, verification is facilitated by the large collection of proof techniques of rewriting: termination, non-termination, confluence, non-confluence, reachability, unreachability, inductive properties, etc. This talk focuses on unreachability properties of a TRS, which entails safety properties on the modeled software system. Starting from a single term s, proving that t is unreachable, i.e., s → * R t is straightforward if R is terminating. This problem is undecidable if R is not terminating or if we consider infinite sets of initial terms s and infinite sets of " Bad " terms t. There exists TRSs classes for which those problems are decidable. For those classes, decidability comes from the fact that the set of reachable terms is regular, i.e., it can be recognized by a tree automaton [5]. Those classes are surveyed in [7]. However, TRSs modeling software systems do not belong to those " decid-able classes " , in general. The rewriting and tree automata community have proposed different techniques to over-approximate the set of reachable terms. Over-approximating reachable terms provide a criterion for unreachability on TRSs and, thus, a criterion for safety of the modeled systems. Those approximation techniques range from TRSs transformation [11], ad hoc automata transformations [6,10,3], CounterExample-Guided Abstraction Refinement (CEGAR) [4,2,1], and abstraction by equational theories [12,9]. I will present the principles underlying those techniques, discuss their pros and cons, and recall some of their applications. Then, I will present a recent attempt to combine abstraction by equational theories and CEGAR to infer accurate over-approximations for TRSs modeling higher-order functional programs [8]

CoLoR: a Coq library on well-founded rewrite relations and its application to the automated verification of termination certificates

Termination is an important property of programs; notably required for programs formulated in proof assistants. It is a very active subject of research in the Turing-complete formalism of term rewriting systems, where many methods and tools have been developed over the years to address this problem. Ensuring reliability of those tools is therefore an important issue. In this paper we present a library formalizing important results of the theory of well-founded (rewrite) relations in the proof assistant Coq. We also present its application to the automated verification of termination certificates, as produced by termination tools