Ana Sofia Gomes

Normative systems represented as hybrid knowledge bases

Normative systems have been advocated as an effective tool to regulate interaction in multi-agent systems. Logic programming rules intuitively correspond to conditional norms, and their semantics is based on the closed world assumption, which allows default negation, often used in norms. However, there are cases where the closed world assumption is clearly not adequate, and others that require reasoning about unknown individuals, which is not possible in logic programming. On the other hand, description logics are based on the open world assumption and support reasoning about unknown individuals, but do not support default negation. In this paper, we demonstrate the need for the aforementioned features (closed and open world assumptions, and reasoning about unknown individuals) in order to model human laws, with examples from the Portuguese Penal Code. We advocate the use of hybrid knowledge bases combining rules and ontologies, which provide the joint expressivity of logic programming and description logics. We define a normative scenario as the pair of a set of facts and a set of norms, and give it a formal semantics by translation into an MKNF knowledge base. We describe the implementation of the language, which computes the relevant consequences of given facts and norms, and use it to establish the resulting sentence in a penal scenario.

Implementing query answering for hybrid MKNF knowledge bases

Ontologies and rules are usually loosely coupled in knowledge representation formalisms. In fact, ontologies use open-world reasoning while the leading semantics for rules use non-monotonic, closed-world reasoning. One exception is the tightly-coupled framework of Minimal Knowledge and Negation as Failure (MKNF), which allows statements about individuals to be jointly derived via entailment from an ontology and inferences from rules. Nonetheless, the practical usefulness of MKNF has not always been clear, although recent work has formalized a general resolution-based method for querying MKNF when rules are taken to have the well-founded semantics, and the ontology is modeled by a general Oracle. That work leaves open what algorithms should be used to relate the entailments of the ontology and the inferences of rules. In this paper we provide such algorithms, and describe the implementation of a query-driven system, CDF-Rules, for hybrid knowledge bases combining both (non-monotonic) rules under the well-founded semantics and a (monotonic) ontology, represented by a CDF (

Transaction logic with external actions

\mathcal{ALCQ}

A goal-directed implementation of query answering for hybrid MKNF knowledge bases

) theory.

External Transaction Logic with Automatic Compensations

We propose External Transaction Logic (or ETR), an extension of Transaction Logic able to represent updates in internal and external domains whilst ensuring a relaxed transaction model. With this aim, ETR deals with two main components: an internal knowledge base where updates follow the strict ACID model, given by the semantics of Transaction Logic; and an external knowledge base of which one has limited or no control and can only execute external actions. When executing actions in the external domain, if a failure occurs, it is no longer possible to simply rollback to the initial state before executing the transaction. For dealing with this, similarly to what is done in databases, we define compensating operations for each external action to be performed to ensure a relaxed model of atomicity and consistency. By executing these compensations in backward order, we obtain a state considered to be equivalent to the initial one.

Combining transactions and automatic repairs

Ontologies and rules are usually loosely coupled in knowledge representation formalisms. In fact, ontologies use open-world reasoning, while the leading semantics for rules use non-monotonic, closed-world reasoning. One exception is the tightly coupled framework of Minimal Knowledge and Negation as Failure (MKNF), which allows statements about individuals to be jointly derived via entailment from ontology and inferences from rules. Nonetheless, the practical usefulness of MKNF has not always been clear, although recent work has formalized a general resolution-based method for querying MKNF when rules are taken to have the well-founded semantics, and the ontology is modeled by a general oracle. That work leaves open what algorithms should be used to relate the entailments of the ontology and the inferences of rules. In this paper we provide such algorithms, and describe the implementation of a query-driven system, CDF-Rules , for hybrid knowledge bases combining both (non-monotonic) rules under the well-founded semantics and a (monotonic) ontology, represented by the Coherent Description Framework Type-1 (

A Procedure for an Event-Condition-Transaction Language

\mathcal{ALCQ}

How to Combine Event Stream Reasoning with Transactions for the Semantic Web

) theory.

A proposal for transactions in the semantic web

External Transaction Logic

Normative systems require hybrid knowledge bases

\mathcal{ETR}