Bounded LTL Model Checking with Stable Models

In this paper bounded model checking of asynchronous concurrent systems is introduced as a promising application area for answer set programming. As the model of asynchronous systems a generalisation of communicating automata, 1-safe Petri nets, are used. It is shown how a 1-safe Petri net and a requirement on the behaviour of the net can be translated into a logic program such that the bounded model checking problem for the net can be solved by computing stable models of the corresponding program. The use of the stable model semantics leads to compact encodings of bounded reachability and deadlock detection tasks as well as the more general problem of bounded model checking of linear temporal logic. Correctness proofs of the devised translations are given, and some experimental results using the translation and the Smodels system are presented.Comment: 32 pages, to appear in Theory and Practice of Logic Programmin

Electromagnetically induced spatial light modulation

We theoretically report that, utilizing electromagnetically induced transparency (EIT), the transverse spatial properties of weak probe fields can be fast modulated by using optical patterns (e.g. images) with desired intensity distributions in the coupling fields. Consequently, EIT systems can function as high-speed optically addressed spatial light modulators. To exemplify our proposal, we indicate the generation and manipulation of Laguerre-Gaussian beams based on either phase or amplitude modulation in hot vapor EIT systems.Comment: 8 pages, 3 figure

Synchronous Counting and Computational Algorithm Design

Consider a complete communication network on $n$ nodes, each of which is a state machine. In synchronous 2-counting, the nodes receive a common clock pulse and they have to agree on which pulses are "odd" and which are "even". We require that the solution is self-stabilising (reaching the correct operation from any initial state) and it tolerates $f$ Byzantine failures (nodes that send arbitrary misinformation). Prior algorithms are expensive to implement in hardware: they require a source of random bits or a large number of states. This work consists of two parts. In the first part, we use computational techniques (often known as synthesis) to construct very compact deterministic algorithms for the first non-trivial case of $f = 1$. While no algorithm exists for $n < 4$, we show that as few as 3 states per node are sufficient for all values $n \ge 4$. Moreover, the problem cannot be solved with only 2 states per node for $n = 4$, but there is a 2-state solution for all values $n \ge 6$. In the second part, we develop and compare two different approaches for synthesising synchronous counting algorithms. Both approaches are based on casting the synthesis problem as a propositional satisfiability (SAT) problem and employing modern SAT-solvers. The difference lies in how to solve the SAT problem: either in a direct fashion, or incrementally within a counter-example guided abstraction refinement loop. Empirical results suggest that the former technique is more efficient if we want to synthesise time-optimal algorithms, while the latter technique discovers non-optimal algorithms more quickly

SAT-based Explicit LTL Reasoning

We present here a new explicit reasoning framework for linear temporal logic (LTL), which is built on top of propositional satisfiability (SAT) solving. As a proof-of-concept of this framework, we describe a new LTL satisfiability tool, Aalta\_v2.0, which is built on top of the MiniSAT SAT solver. We test the effectiveness of this approach by demonnstrating that Aalta\_v2.0 significantly outperforms all existing LTL satisfiability solvers. Furthermore, we show that the framework can be extended from propositional LTL to assertional LTL (where we allow theory atoms), by replacing MiniSAT with the Z3 SMT solver, and demonstrating that this can yield an exponential improvement in performance

Platform Dependent Verification: On Engineering Verification Tools for 21st Century

The paper overviews recent developments in platform-dependent explicit-state LTL model checking.Comment: In Proceedings PDMC 2011, arXiv:1111.006

A SAT-Based Encoding of the One-Pass and Tree-Shaped Tableau System for LTL

A new one-pass and tree-shaped tableau system for LTL sat- isfiability checking has been recently proposed, where each branch can be explored independently from others and, furthermore, directly cor- responds to a potential model of the formula. Despite its simplicity, it proved itself to be effective in practice. In this paper, we provide a SAT-based encoding of such a tableau system, based on the technique of bounded satisfiability checking. Starting with a single-node tableau, i.e., depth k of the tree-shaped tableau equal to zero, we proceed in an incremental fashion. At each iteration, the tableau rules are encoded in a Boolean formula, representing all branches of the tableau up to the current depth k. A typical downside of such bounded techniques is the effort needed to understand when to stop incrementing the bound, to guarantee the completeness of the procedure. In contrast, termination and completeness of the proposed algorithm is guaranteed without com- puting any upper bound to the length of candidate models, thanks to the Boolean encoding of the PRUNE rule of the original tableau system. We conclude the paper by describing a tool that implements our procedure, and comparing its performance with other state-of-the-art LTL solvers

Handling Conflicts in Depth-First Search for LTL Tableau to Debug Compliance Based Languages

Providing adequate tools to tackle the problem of inconsistent compliance rules is a critical research topic. This problem is of paramount importance to achieve automatic support for early declarative design and to support evolution of rules in contract-based or service-based systems. In this paper we investigate the problem of extracting temporal unsatisfiable cores in order to detect the inconsistent part of a specification. We extend conflict-driven SAT-solver to provide a new conflict-driven depth-first-search solver for temporal logic. We use this solver to compute LTL unsatisfiable cores without re-exploring the history of the solver.Comment: In Proceedings FLACOS 2011, arXiv:1109.239

Development of an Agrobacterium transformation system for onion (Allium cepa L.)

Onion (Allium cepa) bulbs of the New Zealand-bred cultivars 'Pukekohe Longkeeper' and 'Early Longkeeper' produced tumourous growths after inoculation with 25 virulent strains of Agrobactenum tumefaciens, A. rubi and A. rhizogenes. The majority of these tumours produced nopaline, indicating that tumour cells were transformed. Some excised tumours produced roots in sterile culture. Eight onion genotypes were screened in tissue culture for callus formation, regeneration of plantlets from callus and clonal multiplication by shoot proliferation. All genotypes could be clonally multiplied and four were readily regenerable from callus. A technique for plantlet multiplication, which uses longitudinally-bisected stems of in vitro-germinated onion seedlings as explants, was developed. Onion (‘Pukekohe Longkeeper', 'Southport White Globe', 'Japanese Saporo Yellow' and 'Hikeeper Fl ') protoplasts were isolated and cultured on a range of media. These protoplasts formed new cell walls and sometimes divided, but only first divisions were regularly seen. Kanamycin, geneticin (G418), hygromycin and chlorsulfuron were evaluated for their use as selective agents in onion transformation experiments. Tissues surveyed for sensitivity to these selective agents included seeds and seedlings on germination and callusing media, established callus on callusing and regeneration media, and shoot cultures on shoot proliferation medium. Hygromycin was shown to be the antibiotic most toxic to tissues of all the surveyed onion cultivars, with effects being obvious in all tissues after 4-5 weeks of culture on concentrations as low as 20 mgl-1. Kanamycin was shown to be the least toxic of the selection agents surveyed. The kanamycin analogue G418 was considerably more toxic to most onion cultures than kanamycin. However, responses of cultures to G418 were slower than those to hygromycin. The herbicide chlorsulfuron was also shown to be toxic to onion seedlings and shoot cultures. The ability of Agrobacterium tumefaciens to transfer foreign genes to A. cepa was demonstrated. A single, putatively transformed plantlet (RC1), was regenerated from an onion seedling stem via callus, following co-cultivation of stem explants with Agrobacterium strain LBA4404 harbouring the binary vector pKIWI110. In addition, 41 auxiliary or adventitious shoots which grew directly from basal plates injected in vitro with four strains of A. tumefaciens (each harbouring the binary vectors pKIWI110 or pGA643) exhibited resistance to G418 in culture. The binary vectors used carry the neomycin phosphotransferase II gene (nptII) controlled by the nopaline synthase (nos) promoter. Both RC1 and some of the shoots growing from basal plate explants produced roots when grown on culture media supplemented with G418. Southern analyses showed that fragments of DNA from RC1 and from five of the 41 G418-resistant shoots hybridized to a 1.25 kbp nptII probe. (β- glucuronidase (GUS) activity was detected in over half of the plantlets derived from basal plate tissue injected with A. tumefaciens strains LBA4404 or C58, both of which harboured pKIWI110. Molecular and phenotypic evidence suggested that the putatively transformed plants produced from injected basal plate tissues were chimeric

An optimal cut-off algorithm for parameterised refinement checking

Abstract The verification of contemporary distributed software systems is challenging, because they are heavily parameterised, containing components whose number and connections cannot be a priori fixed. In this work, we consider the multi-parameterised verification of safety properties by refinement checking in the context of labelled transition systems (LTSs). The LTSs are parameterised by using first-order constructs, sorts, variables, and predicates, while preserving compositionality. This allows us to parameterise not only the number of replicated components but also the communication topology of the system. Our approach to solving a verification task in the parameterised LTS formalism is to determine a finite cut-off set of parameter values such that in order to prove a parameterised system implementation correct with respect to its specification, it is sufficient to consider only finitely many instances of the parameterised system generated by the parameter values in the cut-off set. In the conference version of this work, we converted the problem of determining a finite cut-off set into the unsatisfiability of a first-order formula and provided a satisfiability modulo theories (SMT)-based semi-algorithm for dynamically, i.e., iteratively, computing a cut-off set. In this article, we present a new version of the algorithm and prove that the cut-off sets computed by this new algorithm are optimal. Hence, we call the new version the optimal cut-off algorithm. The algorithm will always terminate for system topologies expressible in the ∃∗∀∗ fragment of first-order logic. It also enables us to consider systems with topologies beyond this fragment, but for these systems, the algorithm is not guaranteed to terminate. We have implemented the approach on top of the Z3 SMT solver and successfully applied it to several system models. As a running example, we consider the leader election phase of the generalised (Byzantine) Raft consensus algorithm and prove the optimal cut-off set of six (respectively, thirteen) parameter values corresponding to instances up to three (respectively, four) servers. To the best of our knowledge, this is the first time a Byzantine variant of the parameterised Raft leader election is automatically verified