This material is based upon work supported by the National
Science Foundation under Grants DMS-8613576, DMS-8902619,
DMS-9205139,
DMS-9504764 and DMS-9803518, an Alfred P. Sloan Foundation
Fellowship (1987-1989),
a joint appointment with the  University of Maryland
Institute for Advanced Computer Studies (1989-1994),
and a Semester Research Award from the General Research Board
of the University of Maryland (Fall 1998).)