Electronic version of an article published as International Journal of Algebra and Computation 2004 Vol. 14 no.5-6 pp.719-739 DOI: 10.1142/S0218196704002043 copyright World Scientific Publishing Company http://ejournals.wspc.com.sg/journals/ijac/mkt/archive.shtml DOI : 10.1142/S0218196704002043Peer reviewe