I. V. Ramakrishnan

Efficient Model Checking Using Tabled Resolution

We demonstrate the feasibility of using the XSB tabled logic programming system as a programmable fixed-point engine for implementing efficient local model checkers. In particular, we present XMC, an XSB-based local model checker for a CCS-like value-passing language and the alternation-free fragment of the modal mu-calculus. XMC is written in under 200 lines of XSB code, which constitute a declarative specification of CCS and the modal mu-calculus at the level of semantic equations.

Efficient access mechanisms for tabled logic programs

Abstract The use of tabling in logic programming allows bottom-up evaluation to be incorporated in a top-down framework, combining advantages of both. At the engine level, tabling also introduces issues not present in pure top-down evaluation, due to the need for subgoals and answers to access tables during resolution. This article describes the design, implementation, and experimental evaluation of data structures and algorithms for high-performance table access. Our approach uses tries as the basis for tables. Tries, a variant of discrimination nets, provide complete discrimination for terms, and permit a lookup and possible insertion to be performed in a single pass through a term. In addition, a novel technique of substitution factoring is proposed. When substitution factoring is used, the access cost for answers is proportional to the size of the answer substitution, rather than to the size of the answer itself. Answer tries can be implemented both as interpreted structures and as compiled WAM-like code. When they are compiled, the speed of computing substitutions through answer tries is competitive with the speed of unit facts compiled or asserted as WAM code. Because answer tries can also be created an order of magnitude more quickly than asserted code, they form a promising alternative for representing certain types of dynamic code, even in Prolog systems without tabling.

Hearsay: enabling audio browsing on hypertext content

In this paper we present HearSay, a system for browsing hypertext Web documents via audio. The HearSay system is based on our novel approach to automatically creating audio browsable content from hypertext Web documents. It combines two key technologies: (1) automatic partitioning of Web documents through tightly coupled structural and semantic analysis, which transforms raw HTML documents into semantic structures so as to facilitate audio browsing; and (2) VoiceXML, an already standardized technology which we adopt to represent voice dialogs automatically created from the XML output of partitioning. This paper describes the software components of HearSay and presents an initial system evaluation.

OntoMiner: bootstrapping and populating ontologies from domain-specific Web sites

Key to the Semantic Web idea are ontologies that can transform legacy HTML documents into Semantic Web documents and encode domain knowledge to facilitate automated reasoning. The techniques presented in the paper can help bootstrap and populate specialized domain ontologies.

CTR-S: a logic for specifying contracts in semantic web services

A requirements analysis in the emerging field of Semantic Web Services (SWS) (see http://daml.org/services/swsl/requirements/) has identified four major areas of research: intelligent service discovery, automated contracting of services, process modeling, and service enactment. This paper deals with the intersection of two of these areas: process modeling as it pertains to automated contracting. Specifically, we propose a logic, called CTR-S,which captures the dynamic aspects of contracting for services.Since CTR-S is an extension of the classical first-order logic, it is well-suited to model the static aspects of contracting as well. A distinctive feature of contracting is that it involves two or more parties in a potentially adversarial situation. CTR-S is designed to model this adversarial situation through its novel model theory, which incorporates certain game-theoretic concepts. In addition to the model theory, we develop a proof theory for CTR-S and demonstrate the use of the logic formodeling and reasoning about Web service contracts.

Automatic annotation of content-rich HTML documents: structural and semantic analysis

Although RDF/XML has been widely recognized as the standard vehicle for representing semantic information on the Web, an enormous amount of semantic data is still being encoded in HTML documents that are designed primarily for human consumption and not directly amenable to machine processing. This paper seeks to bridge this semantic gap by addressing the fundamental problem of automatically annotating HTML documents with semantic labels. Exploiting a key observation that semantically related items exhibit consistency in presentation style as well as spatial locality in template-based content-rich HTML documents, we have developed a novel framework for automatically partitioning such documents into semantic structures. Our framework tightly couples structural analysis of documents with semantic analysis incorporating domain ontologies and lexical databases such as WordNet. We present experimental evidence of the effectiveness of our techniques on a large collection of HTML documents from various news portals.

XMC: A Logic-Programming-Based Verification Toolset

XMC is a toolset for specifying and verifying concurrent systems. Its main mode of verification is temporal-logic model checking [CES86], although equivalence checkers have also been implemented. In its current form, temporal properties are specified in the alternation-free fragment of the modal mu-calculus [Koz83], and system models are specified in XL, a value-passing language based on CCS [Mil89]. The core computational components of the XMC system, such as those for compiling the specification language, model checking, etc., are built on top of the XSB tabled logic-programming system [XSB99].

Logic Programming and Model Checking

We report on the current status of the LMC project, which seeks to deploy the latest developments in logic-programming technology to advance the state of the art of system specification and verification. In particular, the XMC model checker for value-passing CCS and the modal mu-calculus is discussed, as well as the XSB tabled logic programming system, on which XMC is based. Additionally, several ongoing efforts aimed at extending the LMC approach beyond traditional finite-state model checking are considered, including compositional model checking, the use of explicit induction techniques to model check parameterized systems, and the model checking of real-time systems. Finally, after a brief conclusion, future research directions are identified.

Fighting Livelock in the i-Protocol: A Comparative Study of Verification Tools

The i-protocol, an optimized sliding-window protocol for GNU UUCP, came to our attention two years ago when we used the Concurrency Factorys local model checker to detect, locate, and correct a non-trivial livelock in version 1.04 of the protocol. Since then, we have repeated this verification effort with five widely used model checkers, namely, COSPAN, Murϕ, SMV, Spin, and XMC. It is our contention that the i-protocol makes for a particularly compelling case study in protocol verification and for a formidable benchmark of verification-tool performance, for the following reasons: 1) The i-protocol can be used to gauge a tools ability to detect and diagnose livelock errors. 2) The size of the i-protocols state space grows exponentially in the window size, and the entirety of this state space must be searched to verify that the protocol, with the livelock error eliminated, is deadlock- or livelock-free. 3) The i-protocol is an asynchronous, low-level software system equipped with a number of optimizations aimed at minimizing control-message and retransmission overhead. It lacks the regular structure that is often present in hardware designs. In this sense, it provides any verification tool with a vigorous test of its analysis capabilities.

Term matching on parallel computers

Term matching is an important problem that arises very often in term rewriting and in functional and equational programming. In this paper, we present a new parallel algorithm for the term-matching problem on the EREW (Exclusive Read Exclusive Write) model of parallel computation. Our algorithm assumes a string representation of the two terms as its input. The string representation is first transformed into two labeled ordered trees and term matching is then performed on these two trees. If n is the length of the input terms then for any constant ∈, (0<#x2208;≤1) our algorithm uses O(n1−∈) processors and takes O(nelog n) time. If e=0 the same algorithm will run in O(log2n) time. The only other known parallel algorithm for this problem is due to Dwork, Kanellakis and Stockmeyer that requires O(n2) processors and takes either O(log n) or O(log2n) time. However, their algorithm uses the stronger CREW (Concurrent Read Exclusive Write) model of parallel computation and assumes a DAG (Directed Acyclic Graph) representation of the two terms as its input. The techniques used in our algorithm are novel and have wide applicability.