The authors would like to thank Thomas Eiter, the editor in charge for this paper, and the anonymous reviewers for their helpful comments. We would like also to thank Dimitris Vrakas for his careful reading and suggestions in the final version of the paper. Finally, we would like to thank the researchers of the planning community for making their planners available, and more specifically Blai Bonet, Hector Geffner, Joerg Hoffman, Bernhard Nebel, Derek Long, Maria Fox, Romeo Sanchez Nigenda, XuanLong Nguyen and Subbarao Kambhampati.
Ioannis
Refanidis
14-8-2001