Franziska Biegler Mark Daley Markus Holzer Ian McQuillan On the uniqueness of shuffle on words and finite languages. 3711-3724 2009 410 Theor. Comput. Sci. 38-40 http://dx.doi.org/10.1016/j.tcs.2009.04.016 db/journals/tcs/tcs410.html#BieglerDHM09