1. Chandra, A.K., Kozen, D., Stockmeyer, L.J.: Alternation. Journal of ACM 28, 114–133 (1981)
2. Jantzen, M.: On the hierarchy of Petri net languages. R.A.I.R.O. Informatique théorique/Theoretical Informatics 13(1), 19–30 (1979)
3. Joshi, A., Schabes, Y.: Tree-adjoining grammars. In: Rozenberg, G., Salomaa, A. (eds.) Handbook of Formal Languages, vol. 3, pp. 69–124. Springer, Berlin (1997)
4. Kozen, D.: Theory of Computation. Springer, New York (2006)
5. Salomaa, A.: Theory of Automata. Pergamon Press, Oxford (1969)