Stefania Costantini

The DALI Logic Programming Agent-Oriented Language

DALI [3], [2] is an Active Logic Programming Language designed in the line of [6] for executable specification of logical agents. A DALI agent is a logic program that contains a particular kind of rules, reactive rules, aimed at interacting with an external environment. The reactive and proactive behavior of a DALI agent is triggered by several kinds of events: external, internal, present and past events. All the events and actions are timestamped, so as to record when they occurred. The new syntactic entities, i.e., predicates related to events and proactivity, are indicated with special postfixes (which are coped with by a pre-processor) so as to be immediately recognized while looking at a program.

DALICA: Agent-Based Ambient Intelligence for Cultural-Heritage Scenarios

Villa Adriana is an enormous archaeological area where ancient artifacts and modern technology have found an unexpected equilibrium. The old artifacts are huge stone monuments; the modern technology includes PDAs and signals from the Galileo satellite combined with intelligent software agents. As part of the European CUSPIS project (Cultural Heritage Space Identification System, www. cuspis-project.info), we exploited intelligent agents for two ambient-intelligence scenarios applicable to Villa Adriana. One scenario involves cultural assets fruition-the possibility of accessing and enjoying cultural assets. This scenario concerns the dissemination of information about cultural assets; for example, users can visit a museum or archaeological site and receive on their mobile devices appropriate, personalized information about that place. The other scenario involves cultural assets monitoring, which concerns securely transporting cultural assets from the owner organization to a renter organization and back.

Meta-reasoning: A Survey

We present the basic principles and possible applications of systems capable of meta-reasoning and reflection. After a discussion of the seminal approaches, we outline our own perception of the state of the art, mainly but not only in computational logic and logic programming. We review relevant successful applications of meta-reasoning, and the basic underlying semantic principles.

About declarative semantics of logic-based agent languages

In this paper we cope with providing an approach to declarative semantics of logic-based agent-oriented languages, taking then as a case-study the language DALI which has been previously defined by the authors. This “evolutionary semantics” does not resort to a concept of state: rather, it models reception of events as program transformation steps, that produce a “program evolution” and a corresponding “semantic evolution”. Communication among agents and multi-agent systems is also taken into account. The aim is that of modeling agents evolution according to either external (environmental) or internal changes in a logical way, thus allowing in principle the adoption of formal verification methods. We also intend to create a common ground for relating and comparing different approaches/languages.

On the existence of stable models of non-stratified logic programs

In this paper we analyze the relationship between cyclic definitions and consistency in Gelfond-Lifschitzs answer sets semantics (originally defined as ‘stable model semantics’). This paper introduces a fundamental result, which is relevant for Answer Set programming, and planning. For the first time since the definition of the stable model semantics, the class of logic programs for which a stable model exists is given a syntactic characterization. This condition may have a practical importance both for defining new algorithms for checking consistency and computing answer sets, and for improving the existing systems. The approach of this paper is to introduce a new canonical form (to which any logic program can be reduced to), to focus the attention on cyclic dependencies. The technical result is then given in terms of programs in canonical form (canonical programs), without loss of generality: the stable models of any general logic program coincide (up to the language) to those of the corresponding canonical program. The result is based on identifying the cycles contained in the program, showing that stable models of the overall program are composed of stable models of suitable sub-programs, corresponding to the cycles, and on defining the Cycle Graph. Each vertex of this graph corresponds to one cycle, and each edge corresponds to one handle, which is a literal containing an atom that, occurring in both cycles, actually determines a connection between them. In fact, the truth value of the handle in the cycle where it appears as the head of a rule, influences the truth value of the atoms of the cycle(s) where it occurs in the body. We can therefore introduce the concept of a handle path, connecting different cycles. Cycles can be even, if they consist of an even number of rules, or vice versa they can be odd. Problems for consistency, as it is well-known, originate in the odd cycles. If for every odd cycle we can find a handle path with certain properties, then the existence of stable model is guaranteed. We will show that based on this results new classes of consistent programs can be defined, and that cycles and cycle graphs can be generalized to components and component graphs.

Answer Set Programming with Resources

In this article, we propose an extension of Answer Set Programming (ASP) to support declarative reasoning on consumption and production of resources. We call the proposed extension RASP, standing for ‘Resourced ASP’. Resources are modeled by introducing special atoms, called amount-atoms, to which we associate quantities that represent the available amount of a certain resource. The ‘firing’of aRASP rule involving amount-atoms can both consume and produce resources. A RASP rule can be fired several times, according to its definition and to the available quantities of required resources. We define the semantics for RASP programs by extending the usual answer set semantics. Different answer sets correspond to different possible allocations of available resources. We then propose an implementation based on standard ASP-solvers. The implementation consists of a standard translation of each RASP rule into a set of plain ASP-rules and of an inference engine that manages the firing of RASP rules.

Extending and Implementing RASP

In previous work we have proposed an extension to ASP (Answer Set Programming), called RASP, standing for ASP with Resources. RASP supports declarative reasoning on production and consumption of (amounts of) resources. The approach combines answer set semantics with quantitative reasoning and relies on an algebraic structure to support computations and comparisons of amounts. The RASP framework provides some form of preference reasoning on resources usage. In this paper, we go further in this direction by introducing expressive constructs for supporting complex preferences specification on aggregate resources. We present a refinement of the semantics of RASP so as to take into account the new constructs. For all the extensions, we provide an encoding into plain ASP. We prove that the complexity of establishing the existence of an answer set, in such an enriched framework, remains NP-complete as in ASP. Finally, we report on raspberry, a prototypical implementation of RASP. This tool consists of a compiler that, given a ground RASP program, produces a pure ASP encoding suitable to be processed by commonly available ASP-solvers.

A Multi-layered General Agent Model

We propose a layered representation of general agent models with a base layer, composed of basic agent features including control, and a higher layer, consisting of a meta-control with the task of tuning, supervising and modifying the base layer. This provides higher flexibility in how an agent is built and may evolve.

Semantics of a Metalogic Programming Language

This paper presents the declarative and procedural semantics of Reflective Prolog, a new logic language able to represent metaknowledge and use it in the proof process via an extended resolution procedure including forms of implicit reflection. The declarative semantics of a Reflective Prolog definite program is provided in terms of the Least Reflective Herbrand Model of the program, characterized by means of a suitable mapping. The extended resolution is then shown to be sound and complete with respect to the Least Reflective Herband Model.

Complex reactivity with preferences in rule-based agents

In this paper, we extend our previous work on complex reaction in rule-based logical agents. In particular, we introduce the possibility of defining and exploiting complex preferences for choosing the course of action to undertake in response to external events, also based upon a (simplified) form of modal reasoning and on sequences of past events.