Acknowledgments

We would like to thank professor Erez Zadok for his numerous suggestions and those anonymous USENIX reviewers for their invaluable comments. This research is supported by an NSF Career Award MIP-9502067, NSF MIP-9710622, NSF IRI-9711635, NSF EIA-9818342, NSF ANI-9814934, a contract 95F138600000 from Community Management Staff's Massive Digital Data System Program, USENIX student research grants, as well as fundings from Sandia National Laboratory, Reuters Information Technology Inc., and Computer Associates/Cheyenne Inc.