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Visibly pushdown languages
, 2004
"... Abstract. We study congruences on words in order to characterize the class of visibly pushdown languages (Vpl), a subclass of contextfree languages. For any language L, we define a natural congruence on words that resembles the syntactic congruence for regular languages, such that this congruence i ..."
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Cited by 189 (19 self)
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Abstract. We study congruences on words in order to characterize the class of visibly pushdown languages (Vpl), a subclass of contextfree languages. For any language L, we define a natural congruence on words that resembles the syntactic congruence for regular languages, such that this congruence is of finite index if, and only if, L is a Vpl. We then study the problem of finding canonical minimal deterministic automata for Vpls. Though Vpls in general do not have unique minimal automata, we consider a subclass of VPAs called kmodule singleentry VPAs that correspond to programs with recursive procedures without input parameters, and show that the class of wellmatched Vpls do indeed have unique minimal kmodule singleentry automata. We also give a polynomial time algorithm that minimizes such kmodule singleentry VPAs. 1 Introduction The class of visibly pushdown languages (Vpl), introduced in [1], is a subclassof contextfree languages accepted by pushdown automata in which the input letter determines the type of operation permitted on the stack. Visibly pushdown languages are closed under all boolean operations, and problems such as inclusion, that are undecidable for contextfree languages, are decidable for Vpl. Vpls are relevant to several applications that use contextfree languages suchas the modelchecking of software programs using their pushdown models [13]. Recent work has shown applications in other contexts: in modeling semanticsof effects in processing XML streams [4], in game semantics for programming languages [5], and in identifying larger classes of pushdown specifications thatadmit decidable problems for infinite games on pushdown graphs [6].
Logic and databases: a deductive approach
 ACM Computing Surveys
, 1984
"... The purpose of this paper is to show that logic provides a convenient formalism for studying classical database problems. There are two main parts to the paper, devoted respectively to conventional databases and deductive databases. In the first part, we focus on query languages, integrity modeling ..."
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Cited by 170 (2 self)
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The purpose of this paper is to show that logic provides a convenient formalism for studying classical database problems. There are two main parts to the paper, devoted respectively to conventional databases and deductive databases. In the first part, we focus on query languages, integrity modeling and maintenance, query optimization, and data
Games that agents play: A formal framework for dialogues between autonomous agents
 Journal of Logic, Language and Information
, 2001
"... We present a logicbased formalism for modeling of dialogues between intelligent and autonomous software agents, building on a theory of abstract dialogue games which we present. The formalism enables representation of complex dialogues as sequences of moves in a combination of dialogue games, and a ..."
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Cited by 151 (42 self)
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We present a logicbased formalism for modeling of dialogues between intelligent and autonomous software agents, building on a theory of abstract dialogue games which we present. The formalism enables representation of complex dialogues as sequences of moves in a combination of dialogue games, and allows dialogues to be embedded inside one another. The formalism is computational and its modular nature enables dierent types of dialogues to be represented.
Time and time again: The many ways to represent time
 International Journal of Intelligent Systems
, 1991
"... issues remain essentially the same. One of the most crucial problems in any computer system that involves representing the world is the representation of time. This includes applications such as databases, simulation, expert systems and applications of Artificial Intelligence in general. In this bri ..."
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Cited by 135 (0 self)
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issues remain essentially the same. One of the most crucial problems in any computer system that involves representing the world is the representation of time. This includes applications such as databases, simulation, expert systems and applications of Artificial Intelligence in general. In this brief paper, I will give a survey of the basic techniques available for representing time, and then talk about temporal reasoning in a general setting as needed in AI applications. Quite different representations of time are usable depending on the assumptions that can be made about the temporal information to be represented. The most crucial issue is the degree of certainty one can assume. Can one assume that a time stamp can be assigned to each event, or barring that, that the events are fully ordered? Or can we only assume that a partial ordering of events is known? Can events be simultaneous? Can they overlap in time and yet not be simultaneous? If they are not instantaneous, do we know the durations of events? Different answers to each of these questions allow very different representations of time. I. Representations Based on Dating Schemes A good representation of time for instantaneous events, if it is possible, is using an absolute dating system. This involves time stamping each event with an absolute realtime, say taken off the system clock
Logics of communication and change
 Information and Computation
, 2005
"... Current dynamic epistemic logics often become cumbersome and opaque when common knowledge is added for groups of agents. Still, postconditions regarding common knowledge express the essence of what communication achieves. We propose new systems that extend the underlying static epistemic languages i ..."
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Cited by 123 (54 self)
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Current dynamic epistemic logics often become cumbersome and opaque when common knowledge is added for groups of agents. Still, postconditions regarding common knowledge express the essence of what communication achieves. We propose new systems that extend the underlying static epistemic languages in such a way that completeness proofs for the full dynamic systems can be obtained by perspicuous reduction axioms. Also, we include factual alteration, rather than just information change, which allows us to cover a much wider range of phenomena in the area of communication and change. 1
Adding nesting structure to words
 In Developments in Language Theory, LNCS 4036
, 2006
"... We propose the model of nested words for representation of data with both a linear ordering and a hierarchically nested matching of items. Examples of data with such dual linearhierarchical structure include executions of structured programs, annotated linguistic data, and HTML/XML documents. Neste ..."
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Cited by 119 (17 self)
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We propose the model of nested words for representation of data with both a linear ordering and a hierarchically nested matching of items. Examples of data with such dual linearhierarchical structure include executions of structured programs, annotated linguistic data, and HTML/XML documents. Nested words generalize both words and ordered trees, and allow both word and tree operations. We define nested word automata—finitestate acceptors for nested words, and show that the resulting class of regular languages of nested words has all the appealing theoretical properties that the classical regular word languages enjoys: deterministic nested word automata are as expressive as their nondeterministic counterparts; the class is closed under union, intersection, complementation, concatenation, Kleene*, prefixes, and language homomorphisms; membership, emptiness, language inclusion, and language equivalence are all decidable; and definability in monadic second order logic corresponds exactly to finitestate recognizability. We also consider regular languages of infinite nested words and show that the closure properties, MSOcharacterization, and decidability of decision problems carry over. The linear encodings of nested words give the class of visibly pushdown languages of words, and this class lies between balanced languages and deterministic contextfree languages. We argue that for algorithmic verification of structured programs, instead of viewing the program as a contextfree language over words, one should view it as a regular language of nested words (or equivalently, a visibly pushdown language), and this would allow model checking of many properties (such as stack inspection, prepost conditions) that are not expressible in existing specification logics. We also study the relationship between ordered trees and nested words, and the corresponding automata: while the analysis complexity of nested word automata is the same as that of classical tree automata, they combine both bottomup and topdown traversals, and enjoy expressiveness and succinctness benefits over tree automata. 1
Algebraic Reasoning for Probabilistic Concurrent Systems
 Proc. IFIP TC2 Working Conference on Programming Concepts and Methods
, 1990
"... We extend Milner's SCCS to obtain a calculus, PCCS, for reasoning about communicating probabilistic processes. In particular, the nondeterministic process summation operator of SCCS is replaced with a probabilistic one, in which the probability of behaving like a particular summand is given exp ..."
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Cited by 115 (6 self)
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We extend Milner's SCCS to obtain a calculus, PCCS, for reasoning about communicating probabilistic processes. In particular, the nondeterministic process summation operator of SCCS is replaced with a probabilistic one, in which the probability of behaving like a particular summand is given explicitly. The operational semantics for PCCS is based on the notion of probabilistic derivation, and is given structurally as a set of inference rules. We then present an equational theory for PCCS based on probabilistic bisimulation, an extension of Milner's bisimulation proposed by Larsen and Skou. We provide the first axiomatization of probabilistic bisimulation, a subset of which is relatively complete for finitestate probabilistic processes. In the probabilistic case, a notion of processes with almost identical behavior (i.e., with probability 1 \Gamma ffl, for ffl sufficiently small) appears to be more useful in practice than a notion of equivalence, since the latter is often too restricti...
Automatic composition of transitionbased semantic web services with messaging”,
 in Proceedings of VLDB,
, 2005
"... Abstract: In this paper we present Colombo, a framework in which web services are characterized in terms of (i) the atomic processes (i.e., operations) they can perform; (ii) their impact on the "real world" (modeled as a relational database); (iii) their transitionbased behavior; and (i ..."
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Cited by 108 (19 self)
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Abstract: In this paper we present Colombo, a framework in which web services are characterized in terms of (i) the atomic processes (i.e., operations) they can perform; (ii) their impact on the "real world" (modeled as a relational database); (iii) their transitionbased behavior; and (iv) the messages they can send and receive (from/to other web services and "human" clients). As such, Colombo combines key elements from the standards and research literature on (semantic) web services. Using Colombo, we study the problem of automatic service composition (synthesis) and devise a sound, complete and terminating algorithm for building a composite service. Specifically, the paper develops (i) a technique for handling the data, which ranges over an infinite domain, in a finite, symbolic way, and (ii) a technique to automatically synthesize composite web services, based on Propositional Dynamic Logic.