Affiliation: | (1) Department of Computer Science, University of Manchester, M13 9PL Manchester, U.K.;(2) Present address: Department of Computer Science, Royal Holloway & Bedford New College, University of London, Egham Hill, TW20 0EX Egham, Surrey, U.K.;(3) Present address: Laboratory for Computer Science, MIT, 545 Technology Square, 01239 Cambridge, MA, U.S.A. |