Tak Wah Lam Tsuen-Wan Ngan Kar-Keung To Prudence W. H. Wong 148-157 2004 91 Electr. Notes Theor. Comput. Sci. http://dx.doi.org/10.1016/j.entcs.2003.12.010 db/journals/entcs/entcs91.html#LamNTW04