Acknowledgements

The authors wish to thank Amol D. Mali, Eric Lambrecht, Eric Parker, and the anonymous reviewers for their helpful comments on earlier versions of this paper. Thanks are due to Terry Zimmerman for providing insight into UCPOP+EBL. This research is supported in part by NSF Research Initiation Award IRI-9210997, NSF Young Investigator award IRI-9457634, and the ARPA Planning Initiative grants F30602093-C-0039 (phase II) and F30602-95-C-0247 (phase III).