Volker Diekert Yuri Matiyasevich Anca Muscholl Solving Word Equations modulo Partial Commutations. 215-235 1999 224 Theor. Comput. Sci. 1-2 db/journals/tcs/tcs224.html#DiekertMM99 http://dx.doi.org/10.1016/S0304-3975(98)00313-2