PRACTICAL HANDLING OF EXCEPTION-TAINTED RULES AND INDEPENDENCE INFORMATION IN POSSIBILISTIC LOGIC

This paper provides a survey of possibilistic logic as a simple and ef ficient tool for handling nonmonotonic reasoning, with some emphasis o n algorithmic issues. In our previous works, two well-known nonmonoton ic systems have been encoded in the possibility theory framework: the preferential inference based on System P, and the rational closure inf erence proposed by Lehmann and Magidor which relies on System P augmen ted with a rational monotony postulate. System P is known to provide r easonable but very cautious conclusions, and in particular, preferenti al inference is blocked by the presence of ''irrelevant'' properties. When using Lehmann's rational closure, the inference machinery, which is then more productive, may still remain too cautious, or on the cont rary, provide counter-intuitive conclusions. The paper proposes an app roach to overcome the cautiousness of System P and the problems encoun tered by the rational closure inference. This approach takes advantage of (contextual) independence assumptions of the form: the fact that g amma is true (or is false) does not affect the validity of the rule '' normally if alpha then beta''. The modelling of such independence assu mptions is discussed in the possibilistic framework. Moreover, we show that when a counter-intuitive conclusion of a set of defaults can be inferred, it is always possible to repair the set of defaults by addin g suitable information so as to produce the desired conclusions and bl ock unsuitable ones.