Kosaburo Hashiguchi Naoto Sakakibara Shuji Jimbo Equivalence of regular binoid expressions and regular expressions denoting binoid languages over free binoids. 251-266 2004 312 Theor. Comput. Sci. 2-3 http://dx.doi.org/10.1016/j.tcs.2003.09.005 db/journals/tcs/tcs312.html#HashiguchiSJ04