HTN planners no longer have to choose between two precondition semantics: a new compilation folds SHOP-style checks into partially ordered planning with just two facts
In hierarchical task network (HTN) planning, method preconditions mean different things under the SHOP family and under HDDL: the former requires a precondition to hold immediately before the method's first action, the latter only in some earlier state, and under partial order the two can yield different solutions. A new compilation turns the SHOP-style first-action semantics exactly into a standard partially ordered HTN problem without method preconditions, adding only two facts, with a bijection between solutions and a machine-checked proof of language preservation.