ANSWERING ATOMIC QUERIES IN INDEFINITE DEDUCTIVE DATABASES

Answering queries in indefinite systems is a difficult problem both computationally, since it involves non-Horn clauses and factoring, and conceptually, concerning producting beliefs for formulas not derivable from the system. To provide a basis for reasonable beliefs, we propose new criteria as an alternative to the Full Information Principle. Then an approach to producing stable beliefs, called Plausible World Assumption (PWA), is introduced. It is shown how a set of non-Horn clauses can be transformed into a set of so called singleton-head-rules such that evaluation of a given query is reduced to processing of a set of Horn clauses relevant to the query. Finally, algorithms are presented for computing facts and beliefs for atomic queries in accord with the PWA. This method is shown to be more efficient than the known techniques for query evaluation in indefinite systems.