On Debugging of Propositional Answer-Set Programs

Preface This thesis deals with debugging in answer-set programming (ASP), a logic-programming paradigm which became popular in the scientific community for its fully declarative semantics. A major reason why ASP has not found widespread popularity as problem-solving technique yet is a lack of suitable tools for program development. In particular, debugging in ASP is an important field which has not been very well studied so far. This work introduces two new techniques for finding errors in answer-set programs; one is based on ASP-meta-programming, the other relies on a tagging technique. Besides the description of the new approaches, an overview of existing publications concerning debugging of answer-set programs is given. The method using meta-programming constitutes the main contribution of this work. It is based on a transformation of an erroneous program, Π, into the language of a meta-program D M (Π). The encoding allows for extracting exhaustive information about the original program Π. Each answer set of the meta-program describes the behaviour of Π under an interpretation I which is not an answer set of Π. Conversely, for each interpretation I of the original program not belonging to the answer sets of Π, there is a corresponding answer set of the meta-program. The key strategy of our approach is to point out reasons why I is not an answer set of Π. This information can be found in the answer sets of the meta-program, where we distinguish between four kinds of reason: • the body of a rule is satisfied, although no element of the respective rule head is; • an atom a is contained in I, although a is not in the head of any applicable rule; • an integrity constraint is violated under I; or • the truth of an atom in I is caused by itself. The method allows to use answer-set programs in order to formulate complex debugging queries. In particular, the huge amount of information describing an erroneous situation can be reduced to parts relevant to the developer by using standard ASP-optimisation techniques. The second new debugging method is adopted from a tagging technique for transforming ordered logic programs into standard ones. By introducing dedicated atoms, called tags, the applicability of individual rules can be controlled and several properties of the erroneous program can be read off the answer sets of a transformation of the program. A significant feature of both new debugging strategies is their …