Gustav Burosch Hans-Dietrich O. F. Gronau Jean-Marie Laborde Ingo Warnke On posets of m-ary words. 69-91 1996 152 Discrete Mathematics 1-3 http://dx.doi.org/10.1016/0012-365X(94)00256-I db/journals/dm/dm152.html#BuroschGLW96