Learning Hierarchical Problem Networks for Knowledge-Based Planning

In this paper, we review hierarchical problem networks, which encode knowledge about how to decompose planning tasks, and report an approach to learning this expertise from sample solutions. In this framework, procedural knowledge comprises a set of conditional methods that decompose problems – sets of goals – into subproblems. Problem solving involves search through a space of hierarchical plans that achieve top-level goals. Acquisition involves creation of new methods, including state conditions for when they are relevant and goal conditions for when to avoid them. We describe HPNL, a system that learns new methods by analyzing sample hierarchical plans, using violated constraints to identify state conditions and ordering conflicts to determine goal conditions. Experiments with on-line learning in three planning domains demonstrate that HPNL acquires expertise that reduces search on novel problems and examine the importance of learning goal conditions. In closing, we contrast the approach with earlier methods for acquiring search-control knowledge, including explanation-based learning and inductive logic programming. We also discuss limitations and plans for future research.