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09:25  Supported Semantics for Modular Systems SPEAKER: Shahab Tasharrofi ABSTRACT. Modular systems offer a languageindependent declarative framework in which modules from different languages are combined to describe a conceptually and/or computationally complex problem. The original semantics developed to combine modules in a modular system is a modeltheoretical semantics with close ties to relational algebraic operations. In this paper, we introduce a new semantics for modular systems, called supported model semantics, that extends the original modeltheoretic semantics of modular systems and captures nonmonotonic reasoning on the level of modular system (rather than the level of individual modules). Moreover, we also use supported model semantics for modular systems to compare the expressiveness of modular systems with that of heterogeneous nonmonotonic multicontext systems [BE'07]. We show that, under very few conditions, all multicontext systems can be translated into equivalent modular systems. Our result is true for both major semantics of multicontext systems, i.e., the equilibrium semantics and the grounded equilibrium semantics. Thus, we show that modular systems under supported model semantics generalize multicontext systems under either of its major semantics. 
09:50  Infinitary Equilibrium Logic SPEAKER: unknown ABSTRACT. Strong equivalence of logic programs is an important concept in the theory of answer set programming. Equilibrium logic was used to show that propositional formulas are strongly equivalent if and only if they are equivalent in the logic of hereandthere. We extend equilibrium logic to formulas with infinitely long conjunctions and disjunctions, define and axiomatize an infinitary counterpart of the logic of hereandthere, and show that the theorem on strong equivalence holds in the infinitary case as well. 
11:45  Temporal Stable Models are LTLrepresentable SPEAKER: Martín Diéguez ABSTRACT. Many scenarios in Answer Set Programming (ASP) deal with dynamic systems over (potentially infinite) linear time. Temporal Equilibrium Logic (TEL) is a formalism that allows defining the idea of "temporal stable model" not only for dynamic domains in ASP, but aso for any arbitrary theory in the syntax of LinearTime Temporal Logic (LTL). In the past, several tools for computing temporal stable models have been built using wellestablished LTL and automata techniques. These tools displayed the set of temporal stable models of a given theory as a Büchiautomaton and, in many cases, it was also possible to capture such a set by the LTLmodels of a given temporal formula. The fundamental theoretical question of whether this was a general property or not remained open, since it is wellknown that, in general, Büchiautomata are more expressive than LTL. In this paper we show that, indeed, the set of temporal stable models of any arbitrary temporal theory can be succinctly captured as the LTL models of a given temporal formula. 
12:10  Applying Action Language BC with Hierarchical Domain Abstraction to Mobile Robots SPEAKER: unknown ABSTRACT. Action language BC provides an elegant way of formalizing robotic domains which need to be expressed using default logic as well as indirect and recursive action effects. However, generating plans efficiently for large domains using BC can be challenging, even when stateoftheart answer set solvers are used. Since a number of task planning domains in mobile robotics can be easily broken up hierarchically using macroactions, we investigate the computational gains achieved by planning over a hierarchical abstraction of the domain. Each layer of abstraction is described independently using BC, and we plan independently over each description, resolving macroactions as we go down the hierarchy. We present a case study where at least an order of magnitude speedup was achieved in a robot mail collection task by using hierarchical domain abstractions. 
12:35  Action Language BC+: Preliminary Report SPEAKER: Joohyung Lee ABSTRACT. Recently, action language BC, which combines the attractive features of action languages B and C+, was proposed. While BC allows Prologstyle recursive definitions that are not available in C+, it is less expressive than C+ in other ways, such as inability to express nonBoolean and nonexogenous actions. We propose a new action language called BC+, which encompasses all the features of BC and the definite fragment of C+. The syntax of BC+ is identical to the syntax of C+ allowing arbitrary propositional formulas in the causal laws, but its semantics is defined in terms of propositional formulas under the stable model semantics instead of nonmonotonic causal theories. This approach allows many useful ASP constructs, such as choice rules and aggregates, to be directly used in language BC+, and exploits computational methods available in ASP solvers.

14:45  Query Answering in ResourceBased Answer Set Semantics SPEAKER: unknown ABSTRACT. In recent work, we defined ResourceBased Answer Set Semantics, which is an extension to traditional answer set semantics stemming from the study of its relationship with linear logic. In this setting there are no inconsistent programs, and constraints are defined "per se" in a separate layer. In this paper, we propose a queryanswering procedure reminiscent of Prolog for answer set programs under this extended semantics. 
15:10  Declarative Encodings of Acyclicity Properties SPEAKER: unknown ABSTRACT. Many knowledge representation tasks involve trees or similar structures as abstract datatypes. However, devising compact and efficient declarative representations of such structural properties is nonobvious and can be challenging indeed. In this paper, we take a number of acyclicity properties into consideration and investigate various logicbased approaches to encode them. We use answer set programming as the primary representation language but also consider mappings to related formalisms, such as propositional logic, difference logic, and linear programming. We study the compactness of encodings and the resulting computational performance on benchmarks involving acyclic or tree structures. 
15:35  Computing Secure Sets in Graphs using Answer Set Programming SPEAKER: Michael Abseher ABSTRACT. Problems from the area of graph theory always served as fruitful benchmarks in order to explore the performance of Answer Set Programming (ASP) systems. A relatively new branch in graph theory is concerned with socalled secure sets. It is known that verifying whether a set is secure in a graph is already coNPhard. The problem of enumerating all secure sets thus is challenging for ASP and its systems. In particular, encodings for this problem seem to require disjunction and also recursive aggregates. In this paper, we provide such encodings and analyze their performance using the Clingo system. 
16:30  ``Are Preferences Giving You a Headache?''  ``Take asprin!'' SPEAKER: unknown ABSTRACT. In this paper we introduce asprin [1], a general, flexible, and extensible framework for handling preferences among the stable models of a logic program. We show how complex preference relations can be specified through userdefined preference types and their arguments. We describe how preference specifications are handled internally by socalled preference programs which are used for dominance testing. We also give algorithms for computing one, or all, optimal stable models of a logic program. Notably, the algorithms depend on the complexity of the dominance tests and make use of incremental answer set solving technology. [1] asprin stands for ``{AS}P for {Pr}eference handl{in}g''. 
16:55  On the Implementation of Weak Constraints in WASP SPEAKER: unknown ABSTRACT. Optimization problems in Answer Set Programming (ASP) are usually modeled by means of programs with weak constraints. These programs can be handled by algorithms for solving Maximum Satisfiability (MaxSAT) problems, if properly ported to the ASP framework. This paper reports on the implementation of several of these algorithms in the ASP solver WASP, whose empirical analysis highlights pros and cons of different strategies for computing optimal answer sets. 
17:20  Interactive Querybased Debugging of ASP Programs SPEAKER: Kostyantyn Shchekotykhin ABSTRACT. Broad application of answer set programming (ASP) for declarative problem solving requires the development of tools supporting the coding process. Program debugging is one of the crucial activities within this process. Modern ASP debugging approaches allow efficient computation of possible explanations of a fault. However, even for a small program a debugger might return a large number of possible explanations and selection of the correct one must be done manually. In this paper we present an interactive querybased ASP debugging method which extends previous approaches and finds a preferred explanation by means of observations. The system automatically generates a sequence of queries to a programmer asking whether a set of ground atoms must be true in all (cautiously) or some (bravely) answer sets of the program. Since some queries can be more informative than the others, we discuss query selection strategies which, given user's preferences for an explanation, can find the best query. That is, the query an answer of which reduces the overall number of queries required for the identification of a preferred explanation. 
17:45  Computing Answer Sets for Monadic Logic Programs via MapReduce SPEAKER: Ilias Tachmazidis ABSTRACT. In this paper the applicability of the MapReduce framework for parallel computation for monadic logic programs is studied. In monadic programs all predicates are of arity one. Two different approaches are suggested: the first is based on parallelizing on constants, the second parallelizes on predicates. In each case, a method is provided that makes use of MapReduce and calls standard ASP solvers as backends via an abstract API. For each method, we provide a theoretical analysis of its computational impact. 