But It Was True When I Checked: Compiling SHOP-Style Method Preconditions into Partially-Ordered HTN Planning

Harrison Oates

Method preconditions specify when a method may be applied in hierarchical task network (HTN) planning, but their meaning differs under two established semantics. The SHOP planners, like the classical HTN formalism of Erol, Hendler and Nau, require the precondition to hold immediately before the method's first action. HDDL, the input language of the International Planning Competition's HTN track, instead introduces an auxiliary action to check it; under partial order, unrelated actions may invalidate the precondition before the method starts. HDDL's designers adopted this weaker semantics because the first-action reading could not easily be compiled. Subsequent work has repeatedly identified its compilation under partial order as an open question, with existing transformations limited to total order. We give a compilation of the first-action semantics into standard partially ordered HTN planning without method preconditions using only two additional facts, thereby resolving a longstanding question in the HTN planning community.