WASP: European Working group on Answer Set Programming
WP5 Report: Model Applications and Proofs-of-Concept
Participating nodes: TUWIEN (leader), HUT, UMAN, UCY, VUB, BATH.

WP5 Report: Model Applications and Proofs-of-Concept
(draft)

Introduction

After a long period of theoretical research on non-monotonic logic programming, in the recent years several implemented systems have become available, including DLV [1, 2], Smodels [3, 4], NoMoRe [5, 6], ASSAT [7, 8], and Cmodels. These systems provide a computational backbone for the Answer Set Programming (ASP) paradigm [9], a promising approach to declarative problem solving which uses concepts from knowledge representation [10].

The objectives of the workpackage WP5 are

As a starting point, we try to categorize and survey promising application areas giving the focus onto the research which is carried out by members of the European Working group on Answer Set Programming (WASP).

Planning

During the last years, logic programming and answer set programming in particular has been widely accepted as a useful tool for solving classical planning problems by means of suitable transformations [11, 12]. The key issue making ASP an appealing core language for planning problems is indeed the non-monotonic formulation of frame axioms using negation as failure.

The system DLVK, developed at TUWIEN, is a sophisticated planning-frontend to the system DLV. Leone et al. [13] have shown some integrated ad hoc encodings for conformant planning and simple forms of conditional planning, and later extended their work with respect to planning under uncertainty, incomplete information, and action costs, adopting useful concepts of Answer Set Programming such as weak constraints [14,15]. In a joint project TUWIEN and UMAN have proposed the use of DLVK in a planning approach supporting design and monitoring of multi-agent-systems [16].

Another system PAL (Pertinence Action Language) is implemented at UNICORUNA using Smodels as a back-end. Further work within WASP-nodes related to planning is [17,18,19,20,21].

Collected Links:

Preference Reasoning and Advanced Web Data Access

One of the most promising areas for applying ASP in a real-world setting is to provide advanced reasoning services in the context of the Semantic Web. Such services clearly require declarative methods dealing with default and preference information. These requirements are perfectly met by ASP [22] and its extensions for preference reasoning [23, 24, 25, 26], which are supported by several available implementations. We mention here the plp front-end [27, 28] which implements different preference-handling strategies developed in the literature on top of the DLV -engine, the gcplp-system [29], which makes use of graphs following the NoMoRe approach, and the system psmodels, a modification of Smodels that can be used to compute preferred answer sets under the ordered disjunction semantics. Another important core equipment for the required reasoning systems could be so-called open logic programs, in which not all parts of a program are known in advance [30] and thus reflect the inherent incompleteness of the information provided by the WWW. A similar role may be played by taking the notions of strong [31] and uniform equivalence [32, 33, 34] for logic programs into account.

Furthermore, in order to apply ASP for advanced web access the possibility to deal with domain-specific languages, ontologies, as well as dynamic knowledge bases is crucial. In this context there has been remarkable success within the last years [35, 36, 37, 38, 39, 40, 41]. From a more abstract point of view, the work by Heymans and Vermeir [42, 43] extends ASP in order to integrate an expressive class of description logics which are able to play the role of an ontology language, as well as a rule language on the Semantic Web. Further work relating nonmonotonic reasoning principles with the Semantic web includes [44, 45] and is also part of work packages in REWERSE, a recently granted EC Network of Excellence, which is targeted to start in early 2004. Another related project is INFOMIX initiated by the two WASP-nodes TUWIEN and UNICAL together with Universita "La Sapienza" - Roma and the polish company Rodan Systems S.A. The main goal of the INFOMIX project is to provide a set of techniques and associated tools for powerful information integration by using advanced reasoning capabilities, as for instance ASP. Finally, there is plenty of further work going on in the search for information integration tools based on ASP, for instance at WASP-node UNIVAQ, see [46].

Collected Links:

Verification and Configuration

Viewing ASP as basis for a constraint programming paradigm [20] leads to the computation of optimal solutions which may be specified by weights [47, 48]. This allows for numerous further applications. For instance, in product configuration [49] ASP can be used as a declarative semantics providing formal definitions for main concepts in product configuration, including configuration models, requirements and valid configurations. A similar application field is software configuration [50]. This research has led to a prototype configurator for the complete Debian Linux system distribution [51]. Moreover, the important area of symbolic model checking is well suited for ASP as shown in [52, 53, 54, 55].

In particular, HUT is focusing on symbolic model checking and software configuration. An overview about this research is summarized in two projects titled Constraint Programming Based on Default Rules and respectively, Applications of Rule-Based Constraint Programming; both links include basic information about the aforecited topics, a comprehensive list of reference, and links to relevant software). Furthermore, the product configuration research employing ASP techniques for implementation is also done in the product data management group of HUT, where interesting application are emerging, e.g., the WeCoTin project on Web configuration technology with Smodels employed as a core-engine.

Finally, we mention that also the concept of inheritance [56] can be used for configuration tasks, providing a natural representation of reasoning with exceptions. Inheritance is implemented as a frontend within DLV.

Collected Links:

Multi-Agent Systems

There is numerous work going in this area within WASP. Software agents which allow to access information in a heterogeneous information system as provided by sites connected by the Internet (see also above) are an interesting application area for ASP. Furthermore, [57] presents systems of logic programming agents to model the interactions between decision-makers while evolving to a conclusion. Such a system consists of a number of agents connected by means of unidirectional communication channels. Agents communicate with each other by passing answer sets obtained by updating the information received from connected agents with their own private information. At UNIVAQ, an ASP solver is integrated into the DALI language interpreter. DALI is a logic programming language aimed at defining agents and multi-agent systems, with advanced reactive and proactive features. Finally, also UNIRC is involved in joint research within this area, see for instance [58].

Collected Links:

Security and Cryptonanalysis

Formal verification of security protocols has become a key issue in computer security. For instance, in [59, 60] it is shown how security protocols can be specified and verified efficiently and effectively by embedding reasoning about actions into a logic programming language. In particular, Smodels is employed in order to model two significant case studies in protocol verification: the classical Needham-Schroeder public-key protocol, and Aziz-Diffie Key agreement protocol for mobile communication.

In [61], the US Data Encryption Standard (DES) is put forward as an interesting benchmark problem for nonmonotonic reasoning systems, presenting two encodings of DES as logic programs: a direct one out of the standard specifications and an optimized one extending the work of Massacci and Marraro [62]. The computational properties of the ASP-encodings are studied by using them for DES key search with the Smodels-system. Results indicate that the encodings and Smodels are quite competitive: they outperform state-of-the-art SAT-checkers working with an optimized encoding of DES into SAT and are comparable with a SAT-checker that is customized and tuned for the optimized SAT encoding.

Finally we mention that the concept of open logic programs has been shown to be relevant for tasks as policy verification [17].

Collected Links:

Diagnostic Systems and Inconsistency Management

Abductive Logic Programming [63, 64] is widely accepted as a promising approach for diagnostic reasoning tasks. Most research in this area is concerned with the SLDNFA-procedure [65], extending the well known prolog-resolution.

For implementations within the core answer-set paradigm we refer to the diagnosis frontend integrated to the DLV-system, see [66]. One of the most prominent applications for diagnosis using logic programs is a project between Texas Tech University and United Space Alliance on developing a decision support system for the ground controllers of space shuttles [67].

Related to diagnostic reasoning is dealing with inconsistent data per se. In [68] it is shown how DLV can be used to repair inconsistent or incomplete census data. Another approach is followed in [69] where the answer set semantics is extended to deal with inconsistent programs (containing classical negation), by finding a ``best'' answer set. It turns out that this preferred answer set semantics is useful for several applications, for instance database repairs, where minimal repairs are shown to correspond to preferred answer sets. In [70] this approach is further developed by showing that a diagnostic system, both consistency-based and abductive, can be regarded as an ordered logic theory. The preferred answer set semantics nicely fits this intuition: if the observations contradict the normal system behavior, then the semantics will provide an explanation from the fault rules. Further, [71] shows that this approach can be extended to perform abductive reasoning in general, with either a preference relation on the set of abducibles or on the system description itself. The latter case appears naturally in applications such as legal reasoning where rules carry a natural precedence.

Collected Links:

Game Theory and Games

In [72, 73, 74] programs are extended by a new connective representing exclusive disjunction and later by a method to express circumstance-dependent preferences. In the paper the applicability of this extension to several problems from game theory is shown. More specifically, this kind of programs allows for an elegant and intuitive tool to transform finite extensive games with perfect information such that the answer sets of the program correspond, depending on the transformation, to either the Nash equilibria or the subgame perfect equilibria of the game. For some recent developments, see also [75]. Furthermore, the following links provide games(-like) applications for ASP.

Collected Links:

Further Applications

Brewka [76] suggests to use ordered disjunctions [26] as well as abduction techniques [66] for qualitative decision making within the context of ASP. In the literature further applications can be found, including auctions [77], scheduling [78], policy description [79], and workflow management [80]. Finally, we mention the recently presented DLV Java Wrapper [81], a library that "wraps" the DLV-system in an external application, and thus allows to embed ASP-techniques inside object-oriented source code.

Collected Links:

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26
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29
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30
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33
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38
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40
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41
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42
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43
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44
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47
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51
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52
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57
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71
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72
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73
M. de Vos and D. Vermeir.
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74
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75
M. de Vos and D. Vermeir.
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76
G. Brewka.
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77
C. Baral and C. Uyan.
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78
W. Faber, N. Leone, and G. Pfeifer.
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79
E. Bertino, A. Mileo, and A. Provetti.
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80
G. Greco, A. Guzzo, and D. Sacca.
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81
F. Ricca.
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