Acknowledgments

This research is supported by an NSF Career Award MIP-9502067, NSF MIP-9710622, NSF IRI-9711635, NSF EIA-9818342, NSF ANIR-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.