Goal Ordering in Partially Ordered Plans

Partially ordered plans have not solved the goal ordering problem. Consider: a goal in a par­ tially ordered plan is an operator precondition that is not yet achieved; operators, orderings and variable bindings are introduced to achieve such goals. While the planning community has known how to achieve individual goals for some time, there has been little work on the prob­ lem of which one of the many possible goals the planner should achieve next. This paper argues that partially ordered plans do not use­ fully address the goal-ordering problem and then presents a heuristic called temporal coherence which does. Temporal coherence is an ad­ missible heuristic which provides goal-ordering guidance. Temporal coherence is admissible in the sense that if a solution exists in the plan­ ner's search space, then there will be a series of goal achievements permitted by the heuristic which can produce this solution.