1. A.V. Aho and J.D. Ullman, The theory of parsing, translation and compiling, Prentice-Hall, Vol. I, 1972.
2. A.V. Aho and J.D. Ullman, The theory of parsing, translation and compiling, Prentice-Hall, Vol. II, 1973.
3. J.C. Beatty, Two iteration theorems for the LL(k) languages, TCS 12 (1980), 193–228.
4. L. Boasson, Two iteration theorems for some families of languages, JCSS 7 (1973), 583–596.
5. Y. Bar-Hillel, M. Perles and E. Shamir, On formal properties of simple phrase structure grammars, Z. Phonetik Sprachwiss. Kommunikat. 14 (1961), 143–172.