Pasquale Rullo

Disjunctive Stable Models

Disjunctive logic programs have become a powerful tool in knowledge representation and commonsense reasoning. This paper focuses on stable model semantics, currently the most widely acknowledged semantics for disjunctive logic programs. After presenting a new notion of unfounded sets for disjunctive logic programs, we provide two declarative characterizations of stable models in terms of unfounded sets. One shows that the set of stable models coincides with the family of unfounded-free models (i.e., a model is stable iff it contains no unfounded atoms). The other proves that stable models can be defined equivalently by a property of their false literals, as a model is stable iff the set of its false literals coincides with its greatest unfounded set. We then generalize the well-founded WPoperator to disjunctive logic programs, give a fixpoint semantics for disjunctive stable models and present an algorithm for computing the stable models of function-free programs. The algorithms soundness and completeness are proved and some complexity issues are discussed.

Enhancing Disjunctive Datalog by constraints

This paper presents an extension of Disjunctive Datalog (DATALOG/sup V,/spl sim//) by integrity constraints. These are of two types: strong, that is, classical integrity constraints and weak, that is, constraints that are satisfied if possible. While strong constraints must be satisfied, weak constraints express desiderata, that is, they may be violated-actually, their semantics tends to minimize the number of violated instances of weak constraints. Weak constraints may be ordered according to their importance to express different priority levels. As a result, the proposed language (call it, DATALOG/sup V,/spl sim/,c/) is well-suited to represent common sense reasoning and knowledge-based problems arising in different areas of computer science such as planning, graph theory optimizations, and abductive reasoning. The formal definition of the language is first given. The declarative semantics of DATALOG/sup V,/spl sim/,c/ is defined in a general way that allows us to put constraints on top of any existing (model-theoretic) semantics for DATALOG/sup V,/spl sim// programs. Knowledge representation issues are then addressed and the complexity of reasoning on DATALOG/sup V,/spl sim/,c/ programs is carefully determined. An in-depth discussion on complexity and expressiveness of DATALOG/sup V,/spl sim/,c/ is finally reported. The discussion contrasts DATALOG/sup V,/spl sim/,c/ to DATALOG/sup V,/spl sim// and highlights the significant increase in knowledge modeling ability carried out by constraints.

Strong and Weak Constraints in Disjunctive Datalog

This paper presents an extension of disjunctive datalog (Data-log∨,⌝) by integrity constraints. In particular, besides classical integrity constraints (called strong constraints in this paper), the notion of weak constraints is introduced in the language. These are constraints that are satisfied if possible. The semantics of weak constraints tends to minimize the number of violated instances. As a consequence, weak constraints differ from strong constraints only if the latter are unsatisfiable. Weak constraints may be ordered according to their importance to express different priority levels. The formal definition of the semantics of weak constraints is given in a general way that allows to put them on top of any existing (model-theoretic) semantics for Datalog∨,⌝ programs. A number of examples shows that the proposed language (call it Data-log∨,⌝, c) is well-suited to represent complex knowledge-based problems, such as, for instance, NP optimization problems.

A logical framework for reasoning on data access control policies

We propose a logic formalism that naturally supports the encoding of complex security specifications. This formalism relies on a hierarchically structured domain made of subjects, objects and privileges. Authorizations are expressed by logic rules. The formalism supports both negation by failure (possibly unstratified) and true negation. The latter is used to express negative authorizations. It turns out that conflicts may result from a set of authorization rules. Dealing with such conflicts requires the knowledge of the domain structure, such as grantor priorities and object/subject hierarchies, which is used in the deductive process to determine which authorization prevails, if any, on the others. Often, however, conflicts are unsolvable, as they express intrinsic ambiguities. We have devised two semantics as an extension of the well-founded and the stable model semantics of logic programming. We have also defined a number of access policies, each based on two orthogonal choices: one is related to the way of how we cope with multiplicity of authorization sets in case of stable model semantics; the other is concerned with the open/closed assumption. A comparative analysis of the proposed authorization policies, based on their degree of permissivity shows that they form a complete lattice.

COMPLEX: an object-oriented logic programming system

The design and a prototypical implementation of COMPLEX, which is a logic-based system extended with concepts from the object-oriented paradigm and is intended as a tool for the development of knowledge-based applications, are described. The system supports a logic language, called Complex-Datalog (C-Datalog), enhanced by semantic constructs to provide facility for data abstraction. Its implementation is based on a bottom-up computational model that guarantees a fully declarative style of programming. However, the user is also given the possibility of running a query using a top-down model of computation. Efficiency of execution is the result of the integration of different novel technologies for the compilation and the execution of queries. >

An Authorization Model and Its Formal Semantics

This paper describes an advanced authorization mechanism based on a logic formalism. The model supports both positive and negative authorizations. It also supports derivation rules by which an authorization can be granted on the basis of the presence or absence of other authorizations. Subjects, objects and authorization types are organized into hierarchies, supporting a more adequate representation of their semantics. From the authorizations explicitly specified, additional authorizations are automatically derived by the system based on those hierarchies. The combination of all the above features results in a powerful yet flexible access control mechanism.

Effective implementation of negation in database logic query languages

Total stable models provide a powerful semantics for DATALOG− programs which increases the expressive power of current database query language by means of non-determinism. An efficient algorithm for determining one of stable models of a DATALOG− programs, if any, is presented so that stable models may have also a practical interest.

The KIWIS knowledge base management system

This report describes the functionality and architecture of the KIWI system, an advanced knowledge-base environment for large database systems, which is currently being developed within the framework of the ESPRIT programme (P2424) by a consortium of industrial and research organizations consisting of Alacatel-Bell Tel. (BE), CRAI (I), Enidata (I), FORTH (GR), ORIGIN (NL), SISU (SE), the universities of Antwerp UIA (BE, Coordinating Contractor), Calabria (I), and LAquila (I).

Semantics and expressiveness of disjunctive ordered logic

AbstractThe paper proposes a knowledge representation language which extends logic programming with disjunction, inheritance, true negation and modularization. The resulting language is called Disjunctive Ordered Logic (