control, and our research on transformational POCL planning was suspended. With the recent improvements in search control for POCL planners, it **may be worthwhile to once** again consider adding transformational plan operators.

In addition to considering different search control heuristics, we could also have instances of VHPOP **working with lifted actions instead of ground actions. Recent improvements**
to the code have significantly reduced the overhead for maintaining binding constraints, making planning with lifted actions look considerably more **favorable than was reported in**
earlier work (Younes & Simmons, 2002). Planning with lifted **actions could be beneficial for** problems with a high branching factor in the search space due **to a large number of objects.**
We would also like to see support for numeric effects and preconditions in future versions of VHPOP**. This would make** VHPOP **fully compatible with PDDL2.1. We have also**
mentioned the need for plan ranking heuristics better tailored for temporal planning, so that VHPOP**'s performance in terms of plan execution time for domains with durative**
actions can approach the performance of the best temporal planners at IPC3.

## Acknowledgments

This effort was sponsored by the Defense Advanced Research Project Agency (DARPA) and the Army Research Office (ARO), under contract no. DAAD19-01-1-0485. The U.S.

Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright annotation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted **as necessarily representing** the official policies or endorsements, either expressed or implied, of DARPA, ARO, or the U.S. Government.

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