CO-PLAN: Combining SAT-Based Planning with Forward-Search

This extended-abstract introduces CO-PLAN, a two-phase propositional planning system for the cost-optimal case. During the first phase, CO-PLAN constructs the n-step plangraph for increasing values of n, passing the corresponding decision problem at each stage to a modified Boolean satisfiability procedure. This procedure determines whether an nstep plan exists, and if so identifies a plan that has the minimal action cost given the plan length bound n. The second phase proceeds as soon as the first phase yields a plan. This consists of a forward-search in the problem state space, bounded by the action-cost of the best plan found during the first phase.