1. Aho, A. V.: Indexed grammars—an extension of context-free grammars. JACM 15(1968) 647–671.
2. Berstel, J.: Transductions and Context-Free Languages. Teubner, 1979.
3. Berstel, J., Reutenauer, C: Recognizable formal power series on trees. Theor. Comput. Sei. 18(1982) 115–148.
4. Bozapalidis, S.: Alphabetic tree relations. Theoret. Comput. Sei. 99(1992) 177–211.
5. Bozapalidis, S.: Equational elements in additive algebras. Technical Report, Aristotle University of Thessaloniki, 1997.