• The planning Domain Definition Language (pddl) is a modelling language for expressing ai planning problems, and used as the input language of a large number of general-purpose ai planning systems. The role of a plan validator is to check if a plan (generated by an ai planner or manually written) is valid, according to the domain and problem specification. A validator is a very useful tool for debugging a domain/problem specification, a planner implementation, and indeed the specification of pddl itself.