Regular hedge model checking

We extend the regular model checking framework so that it can handle systems with arbitrary width tree-like structures. Con gurations of a system are represented by trees of arbitrary arities, sets of con gurations are represented by regular hedge automata, and the dynamics of a system is modeled by a regular hedge transducer. We consider the problem of computing the transitive closure T + of a regular hedge transducer T. This construction is not possible in general. Therefore, we present a general acceleration technique for computing T+. Our method consists of enhancing the termination of the iterative computation of the different compositions Ti by merging the states of the hedge transducers according to an appropriate equivalence relation that preserves the traces of the transducers. We provide a methodology for effectively deriving equivalence relations that are appropriate. We have successfully applied our technique to compute transitive closures for some mutual exclusion protocols de ned on arbitrary width tree topologies, as well as for an XML application.4th IFIP International Conference on Theoretical Computer ScienceRed de Universidades con Carreras en Informática (RedUNCI

Regular hedge model checking

We extend the regular model checking framework so that it can handle systems with arbitrary width tree-like structures. Con gurations of a system are represented by trees of arbitrary arities, sets of con gurations are represented by regular hedge automata, and the dynamics of a system is modeled by a regular hedge transducer. We consider the problem of computing the transitive closure T + of a regular hedge transducer T. This construction is not possible in general. Therefore, we present a general acceleration technique for computing T+. Our method consists of enhancing the termination of the iterative computation of the different compositions Ti by merging the states of the hedge transducers according to an appropriate equivalence relation that preserves the traces of the transducers. We provide a methodology for effectively deriving equivalence relations that are appropriate. We have successfully applied our technique to compute transitive closures for some mutual exclusion protocols de ned on arbitrary width tree topologies, as well as for an XML application.4th IFIP International Conference on Theoretical Computer ScienceRed de Universidades con Carreras en Informática (RedUNCI

Rewrite Closure and CF Hedge Automata

We introduce an extension of hedge automata called bidimensional context-free hedge automata. The class of unranked ordered tree languages they recognize is shown to be preserved by rewrite closure with inverse-monadic rules. We also extend the parameterized rewriting rules used for modeling the W3C XQuery Update Facility in previous works, by the possibility to insert a new parent node above a given node. We show that the rewrite closure of hedge automata languages with these extended rewriting systems are context-free hedge languages

Rewrite based Verification of XML Updates

We consider problems of access control for update of XML documents. In the context of XML programming, types can be viewed as hedge automata, and static type checking amounts to verify that a program always converts valid source documents into also valid output documents. Given a set of update operations we are particularly interested by checking safety properties such as preservation of document types along any sequence of updates. We are also interested by the related policy consistency problem, that is detecting whether a sequence of authorized operations can simulate a forbidden one. We reduce these questions to type checking problems, solved by computing variants of hedge automata characterizing the set of ancestors and descendants of the initial document type for the closure of parameterized rewrite rules

Two-Way Visibly Pushdown Automata and Transducers

Automata-logic connections are pillars of the theory of regular languages. Such connections are harder to obtain for transducers, but important results have been obtained recently for word-to-word transformations, showing that the three following models are equivalent: deterministic two-way transducers, monadic second-order (MSO) transducers, and deterministic one-way automata equipped with a finite number of registers. Nested words are words with a nesting structure, allowing to model unranked trees as their depth-first-search linearisations. In this paper, we consider transformations from nested words to words, allowing in particular to produce unranked trees if output words have a nesting structure. The model of visibly pushdown transducers allows to describe such transformations, and we propose a simple deterministic extension of this model with two-way moves that has the following properties: i) it is a simple computational model, that naturally has a good evaluation complexity; ii) it is expressive: it subsumes nested word-to-word MSO transducers, and the exact expressiveness of MSO transducers is recovered using a simple syntactic restriction; iii) it has good algorithmic/closure properties: the model is closed under composition with a unambiguous one-way letter-to-letter transducer which gives closure under regular look-around, and has a decidable equivalence problem

Completing Queries: Rewriting of IncompleteWeb Queries under Schema Constraints

Reactive Web systems, Web services, and Web-based publish/ subscribe systems communicate events as XML messages, and in many cases require composite event detection: it is not sufficient to react to single event messages, but events have to be considered in relation to other events that are received over time. Emphasizing language design and formal semantics, we describe the rule-based query language XChangeEQ for detecting composite events. XChangeEQ is designed to completely cover and integrate the four complementary querying dimensions: event data, event composition, temporal relationships, and event accumulation. Semantics are provided as model and fixpoint theories; while this is an established approach for rule languages, it has not been applied for event queries before

First-order logic for safety verification of hedge rewriting systems

In this paper we deal with verification of safety properties of hedge rewriting systems and their generalizations. The verification problem is translated to a purely logical problem of finding a finite countermodel for a first-order formula, which is further tackled by a generic finite model finding procedure. We show that the proposed approach is at least as powerful as the methods using regular invariants. At the same time the finite countermodel method is shown to be efficient and applicable to the wide range of systems, including the protocols operating on unranked trees

Monitoring hedge funds: a fi nancial stability perspective.

Investor inflows into hedge funds have been significant in recent years and they have continued unabated. As a result, the presence and role of these investment funds in global capital markets have become increasingly important, and to a much greater extent than the amount of capital they manage would suggest. This is because hedge funds can, and often do, leverage their investment positions. Indeed, their leveraged assets are sometimes comparable with the assets of large banks. The growing and active participation of hedge funds in a large number of financial markets implies that the functioning of these markets could be seriously affected if the hedge fund sector came under stress. The positive contribution of hedge funds to the efficiency and liquidity of global financial markets is widely recognised, but there are also concerns that in times of stress their activities may create risks to financial stability. The lack of transparency and limited publicly available information about their balance sheets and activities poses significant challenges for financial stability analysis. While it is possible to base such an analysis on a multitude of information sources on hedge fund activities – including dedicated financial media, commercial hedge fund databases, quarterly industry reports, hedge fund return indices, academic studies, some supervisory data and market surveillance – these sources are not sufficient for an adequate monitoring and robust evaluation of hedge fund activities from a financial stability perspective. Three groups of indicators could be important for financial stability analysis, namely those which shed light on banks’ exposures to hedge funds, provide yardsticks of the crowding of hedge fund trades, and facilitate the gauging of endogenous hedge fund vulnerabilities. The latter group would include the measures of funding liquidity risk, leverage and exposures to market risk factors. The construction of all these indicators would be greatly facilitated if basic information on hedge fund balance sheets were available. Since this is not the case, various indirect estimation methods have to be relied upon. A “desirable vs. available” analysis reveals the most important information gaps, but it does not aim at providing recommendations on how to enhance hedge fund transparency in practice. Instead, it proposes three elements which a transparency framework would ideally include: fi rst, more aggregate information to all market participants; second, a highly standardised reporting template that would make disclosures more effective; fi nally, adequate information for a joint analysis of the aggregate activities of banks, hedge funds and other highly leveraged institutions in order to have a comprehensive picture of risks to the smooth functioning of financial markets.