V. Lonati, and M. Pradella. Proceedings of the 34th International Symposium on Mathematical Foundations of Computer Science 2009, page 549--560. Berlin, Heidelberg, Springer-Verlag, (2009)
M. Anselmo, D. Giammarresi, and M. Madonia. Developments in Language Theory, volume 4588 of Lecture Notes in Computer Science, Springer Berlin Heidelberg, (2007)