Eliminating Negative Occurrences of Derived Predicates from PDDL Axioms
This addresses a technical limitation in planning domain definition for AI researchers, but it is incremental as it formalizes an existing practice.
The paper tackles the problem of negative occurrences of derived predicates in PDDL axioms, which are restricted by the standard but often relaxed in practice, and presents a transformation to eliminate them, showing that both variants express the same queries as least fixed-point logic.
Axioms are a feature of the Planning Domain Definition Language PDDL that can be considered as a generalization of database query languages such as Datalog. The PDDL standard restricts negative occurrences of predicates in axiom bodies to predicates that are directly set by actions and not derived by axioms. In the literature, authors often deviate from this limitation and only require that the set of axioms is stratifiable. Both variants can express exactly the same queries as least fixed-point logic, indicating that negative occurrences of derived predicates can be eliminated. We present the corresponding transformation.