We are grateful to Malcolm Pradhan and Gregory Provan for sharing with us the CPCS networks. We also thank Jack Breese, Bruce D'Ambrosio, Mike Horsch, Runping Qi, and Glenn Shafer for valuable discussions, and Ronen Brafman, Chris Geib, Mike Horsch and the anonymous reviewers for very helpful comments. Mr. Tak Yin Chan has been a great help in the experimentations. Research was supported by NSERC Grant OGPOO44121, the Institute for Robotics and Intelligent Systems, Hong Kong Research Council grant HKUST658/95E and Sino Software Research Center grant SSRC95/96.EG01.