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SHOP2 [Nau, Au, Ilghami, Kuter, Murdoch, Wu, YamanNau et al.2003] is a Hierarchical Task Network (HTN) planner. Like most other HTN planners, SHOP2 allows tasks and subtasks to be partially ordered. Thus plans may interleave subtasks from different tasks during expansion of tasks. However, unlike most other HTN planners, SHOP2 generates the steps of each plan in the same order that those steps will later be executed and it can therefore maintain a representation of the current state at each stage in the planning process. This makes it much easier to incorporate substantial expressive power into the HTNs used by SHOP2. For example, they might include axioms, mixed symbolic and numeric computation, or even calls to external programs.

Derek Long 2003-11-06