Block-Structured Plan Deordering

Partially ordered plans have several useful properties, such as exhibiting the structure of the plan more clearly which facilitates post-plan generation tasks like scheduling the plan, explaining it to a user, or breaking it into subplans for distributed execution. The standard interpretation of partial ordering implies that whenever two subplans are unordered, every interleaving of steps from the two forms a valid execution. This restricts deordering to cases where individual steps (i.e., actions) are independent. We propose a weaker notion of partial ordering that divides the plan into blocks, such that the steps in a block may not be interleaved with steps outside the block, but unordered blocks can be executed in any sequence. We present an algorithm to find such deorderable blocks, and show that it enables deordering plans in many cases where no deordering is possible under the standard interpretation.