1. Engelfriet, J., Fülöp, Z., Vogler, H.: Bottom-up and top-down tree series transformations. J. Autom. Lang. Combin. 7, 11–70 (2002)
2. Rounds, W.C.: Mappings and grammars on trees. Math. Systems Theory 4, 257–287 (1970)
3. Thatcher, J.W.: Generalized2 sequential machine maps. J. Comput. System Sci. 4, 339–367 (1970)
4. Berstel, J., Reutenauer, C.: Recognizable formal power series on trees. Theoret. Comput. Sci. 18, 115–148 (1982)
5. Kuich, W.: Formal power series over trees. In: Bozapalidis, S. (ed.) Proc. 3rd Int. Conf. Develop. in Lang. Theory, Aristotle University of Thessaloniki, pp. 61–101 (1998)