∗ School of Oriental and African Studies, University of London, London WC1H OXG, UK. E-mail: [email protected]. Most of the first author's work on this paper was done while he was a Research Staff Member in the Computer Science Department of the IBM T.J. Watson Research Center † Sietec Systemtechnik (Siemens AG), D-13623 Berlin, Germany. E-mail: [email protected] The second author’s work o...
