1. A.V. Aho and J.D. Ullman; Translations on a context free grammar, Inform, and Control 19 (1971), 439–475.
2. R. Bloem, J. Engelfriet; Monadic second order logic and node relations on graphs and trees, in J. Mycielski, G. Rozenberg, and A. Salomaa, editors, Structures in Logic and Computer Science, Lecture Notes in Computer Science 1261 , Springer-Verlag, 1997, pp. 144–161.
3. R. Bloem, J. Engelfriet; Characterization of properties and relations defined in monadic second order logic on the nodes of trees, Tech. Report 97–03, Leiden University, August 1997.
4. M. Blum, C. Hewitt; Automata on a 2-dimensional tape, in Proc. 8th IEEE Symp. on Switching and Automata Theory, pp. 155–160, 1967.
5. M. Blum, D. Kozen; On the power of the compass (or, why mazes are easier to search than graphs), Proc. 19th FOCS (Annual Symposium on Foundations of Computer Science), 1978, pp. 132–142.