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200
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 122 (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
Expressiveness and complexity of XML schema
"... The common abstraction of XML Schema by unranked regular tree languages is not entirely accurate. To shed some light on the actual expressive power of XML Schema, intuitive semantical characterizations of the Element Declarations Consistent (EDC) rule are provided. In particular, it is obtained that ..."
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Cited by 60 (22 self)
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The common abstraction of XML Schema by unranked regular tree languages is not entirely accurate. To shed some light on the actual expressive power of XML Schema, intuitive semantical characterizations of the Element Declarations Consistent (EDC) rule are provided. In particular, it is obtained that schemas satisfying EDC can only reason about regular properties of ancestors of nodes. Hence, w.r.t. expressive power, XML Schema is closer to DTDs than to tree automata. These theoretical results are complemented with an investigation of the XML Schema Definitions (XSDs) occurring in practice, revealing that the extra expressiveness of XSDs over DTDs is only used to a very limited extent. As this might be due to the complexity of the XML Schema specification and the difficulty to understand the effect of constraints on typing and validation of schemas, a simpler formalism equivalent to XSDs is proposed. It is based on contextual patterns rather than on recursive types and it might serve as a lightweight front end for XML Schema. Next, the effect of EDC on the way XML documents can be typed is discussed. It is argued that a cleaner, more robust, larger but equally feasible class is obtained by replacing EDC with the notion of 1pass preorder typing (1PPT): schemas that allow to determine the type of an element of a streaming document when its opening tag is met. This notion can be defined in terms of
Web Service Interfaces
, 2005
"... We present a language for specifying web service interfaces. A web service interface puts three kinds of constraints on the users of the service. First, the interface specifies the methods that can be called by a client, together with types of input and output parameters; these are called signature ..."
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Cited by 52 (2 self)
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We present a language for specifying web service interfaces. A web service interface puts three kinds of constraints on the users of the service. First, the interface specifies the methods that can be called by a client, together with types of input and output parameters; these are called signature constraints. Second, the interface may specify propositional constraints on method calls and output values that may occur in a web service conversation; these are called consistency constraints. Third, the interface may specify temporal constraints on the ordering of method calls; these are called protocol constraints. The interfaces can be used to check, first, if two or more web services are compatible, and second, if a web service A can be safely substituted for a web service B. The algorithm for compatibility checking verifies that two or more interfaces fulfill each others' constraints. The algorithm for substitutivity checking verifies that service A demands fewer and fulfills more constraints than service B.
A robust class of contextsensitive languages
 In LICS
, 2007
"... We define a new class of languages defined by multistack automata that forms a robust subclass of contextsensitive languages, with decidable emptiness and closure under boolean operations. This class, called multistack visibly pushdown languages (MVPLs), is defined using multistack pushdown auto ..."
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Cited by 43 (7 self)
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We define a new class of languages defined by multistack automata that forms a robust subclass of contextsensitive languages, with decidable emptiness and closure under boolean operations. This class, called multistack visibly pushdown languages (MVPLs), is defined using multistack pushdown automata with two restrictions: (a) the pushdown automaton is visible, i.e. the input letter determines the operation on the stacks, and (b) any computation of the machine can be split into�stages, where in each stage, there is at most one stack that is popped. MVPLs are an extension of visibly pushdown languages that captures noncontext free behaviors, and has applications in analyzing abstractions of multithreaded recursive programs, significantly enlarging the search space that can be explored for them. We show that MVPLs are closed under boolean operations, and problems such as emptiness and inclusion are decidable. We characterize MVPLs using monadic secondorder logic over appropriate structures, and exhibit a Parikh theorem for them. 1.
Quantitative analysis of probabilistic pushdown automata: . . .
, 2005
"... Probabilistic pushdown automata (pPDA) have been identified as a natural model for probabilistic programs with rcursive procedure calls. Previous works considered the decidability and complexity of the modelchecking problem for pPDA and various probabilistic temporal logics. In this paper we concen ..."
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Cited by 40 (15 self)
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Probabilistic pushdown automata (pPDA) have been identified as a natural model for probabilistic programs with rcursive procedure calls. Previous works considered the decidability and complexity of the modelchecking problem for pPDA and various probabilistic temporal logics. In this paper we concentrate on computing the expected values and variances of various random variables defined over runs of a given probabilistic pushdown automaton. In particular, we show how to compute the expected accumulated reward and the expected gain for certain classes of reward functions. Using these results, we show how to analyze various quantitative properties of pPDA that are not expressible in conventional probabilistic temporal logics.
Visibly pushdown games
 In FSTTCS 2004
, 2004
"... Abstract. The class of visibly pushdown languages has been recently defined as a subclass of contextfree languages with desirable closure properties and tractable decision problems. We study visibly pushdown games, which are games played on visibly pushdown systems where the winning condition is gi ..."
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Cited by 30 (6 self)
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Abstract. The class of visibly pushdown languages has been recently defined as a subclass of contextfree languages with desirable closure properties and tractable decision problems. We study visibly pushdown games, which are games played on visibly pushdown systems where the winning condition is given by a visibly pushdown language. We establish that, unlike pushdown games with pushdown winning conditions, visibly pushdown games are decidable and are 2Exptimecomplete. We also show that pushdown games against Ltl specifications and Caret specifications are 3Exptimecomplete. Finally, we establish the topological complexity of visibly pushdown languages by showing that they are a subclass of Boolean combinations of Σ3 sets. This leads to an alternative proof that visibly pushdown automata are not determinizable and also shows that visibly pushdown games are determined. 1
Firstorder and temporal logics for nested words
 In LICS 2007
"... Nested words are a structured model of execution paths in procedural programs, reflecting their call and return nesting structure. Finite nested words also capture the structure of parse trees and other treestructured data, such as XML. We provide new temporal logics for finite and infinite nested ..."
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Cited by 26 (4 self)
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Nested words are a structured model of execution paths in procedural programs, reflecting their call and return nesting structure. Finite nested words also capture the structure of parse trees and other treestructured data, such as XML. We provide new temporal logics for finite and infinite nested words, which are natural extensions of LTL, and prove that these logics are firstorder expressivelycomplete. One of them is based on adding a ”within” modality, evaluating a formula on a subword, to a logic CaRet previously studied in the context of verifying properties of recursive state machines. The other logic is based on the notion of a summary path that combines the linear and nesting structures. For that logic, both modelchecking and satisfiability are shown to be EXPTIMEcomplete. Finally, we prove that firstorder logic over nested words has the threevariable property, and we present a temporal logic for nested words which is complete for the twovariable fragment of firstorder. 1
The Tree Width of Auxiliary Storage
"... We propose a generalization of results on the decidability of emptiness for several restricted classes of sequential and distributed automata with auxiliary storage (stacks, queues) that have recently been proved. Our generalization relies on reducing emptiness of these automata to finitestate grap ..."
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Cited by 24 (2 self)
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We propose a generalization of results on the decidability of emptiness for several restricted classes of sequential and distributed automata with auxiliary storage (stacks, queues) that have recently been proved. Our generalization relies on reducing emptiness of these automata to finitestate graph automata (without storage) restricted to monadic secondorder (MSO) definable graphs of bounded treewidth, where the graph structure encodes the mechanism provided by the auxiliary storage. Our results outline a uniform mechanism to derive emptiness algorithms for automata, explaining and simplifying several existing results, as well as proving new decidability results. Categories and Subject Descriptors F.1.1 [Theory of Computation]:
Thirdorder Idealized Algol with iteration is decidable
 TCS
, 2008
"... Abstract. The problems of contextual equivalence and approximation are studied for the thirdorder fragment of Idealized Algol with iteration (IA ∗ 3). They are approached via a combination of game semantics and language theory. It is shown that for each IA ∗ 3term one can construct a pushdown auto ..."
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Cited by 24 (5 self)
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Abstract. The problems of contextual equivalence and approximation are studied for the thirdorder fragment of Idealized Algol with iteration (IA ∗ 3). They are approached via a combination of game semantics and language theory. It is shown that for each IA ∗ 3term one can construct a pushdown automaton recognizing a representation of the strategy induced by the term. The automata have some additional properties ensuring that the associated equivalence and inclusion problems are solvable in Ptime. This gives an Exptime decision procedure for contextual equivalence and approximation for βnormal terms. Exptimehardness is also shown in this case, even in the absence of iteration. 1