A Semantics for HTN Methods

Despite the extensive development of first-principles planning in recent years, planning applications are still primarily developed using knowledge-based planners which can exploit domain-specific heuristics and weaker domain models. Hierarchical Task Network (HTN) planners capture domainspecific heuristics for more efficient search, accommodate incomplete causal models, and can be used to enforce standard operating procedures. Unfortunately, we do not have semantics for the methods or tasks that make up HTN models, that help evaluate the correctness of methods, or to build a reliable executive for HTN plans. This paper fills the gap by providing a well-defined semantics for the methods and plans of SHOP2, a state-of-the-art HTN planner. The semantics are defined in terms of concurrent golog (ConGolog) and the situation calculus. We provide a proof of equivalence between the plans generated by SHOP2 and the action sequences of the ConGolog semantics. We show how the semantics reflects the distinction between plan-time and execution-time, and provide some simple examples showing how the semantics can support method verification. The semantics provide an implementation-neutral specification for an executive, showing how an executive must treat the plans SHOP2 generates in order to enforce the expected behaviors. Future directions include automated verification of method specifications, automatically generating plan monitors, and plan revision and repair.