1. Indexed grammars—an extension of context-free grammars;Aho;JACM,1968
2. Nested stack automata;Aho;JACM,1969
3. A. V. Aho and J. Ullman, private communication.
4. On formal properties of simple phrase structure grammars;Bar-Hillel;Z. Phonetik Sprachwiss Kummunikat,1961
5. Grammars With Macro-Like Productions;Fischer,1968