1. Aho, A. V., and Ullman, J. D. (1972),The Theory of Parsing, Translation, and Compiling, Vol. 1, Prentice-Hall, Englewood Cliffs, N.J.
2. Berstel, J. (1979),Transductions and Context-Free Languages, Teubner, Stuttgart.
3. Bader, C., and Moura, A. (1982), A generalization of Ogden's lemma,J. Assoc. Comput. Mach.,29(2), 404–407.
4. Bar-Hillel, Y., Perles, M., and Shamir, E. (1961), On formal properties of simple phrase structure grammars,Z. Phonetik. Sprachwiss. Komm.,14, 143–172.
5. Boasson, L., and Horváth, S. (1978), On languages satisfying Ogden's lemma,RAIRO Inform. Théor.,12(3), 201–202.