1. Anselmo, M., Restivo, A.: Factorizing languages. In: IFIP Congress (1), pp. 445–450 (1994)
2. Berstel, J.: Transductions and context-free languages. B. G. Teubner (1979)
3. Berstel, J., Perrin, D.: The theory of codes, vol. 117. Academic Press, London (1985)
4. Cassaigne, J., Karhumäki, J., Salmela, P.: The conjugacy of biprefix sets (to appear)
5. Choffrut, C., Harju, T., Karhumäki, J.: A note on decidability questions on presentations of word semigroups. Theor. Comput. Sci. 183(1), 83–92 (1997)