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2. Generalizations of regular events;Amar;Inform. Control,1965
3. On formal properties of simple phrase structure grammars;Bar-Hillel;Z. Phonetik, Sprachwiss., Kommunikationforsch.,1961
4. The algebraic theory of context-free languages;Chomsky,1963
5. On relations defined by generalized finite automata;Elgot;IBM J. Res. Develop.,1965