1. Recursive procedures;De Bakker,1971
2. Equations in a space of languages;Blikle,1971
3. The algebraic theory of context free languages;Chomsky,1963
4. The Mathematical Theory of Context Free Languages;Ginsburg,1966
5. Two families of languages related to ALGOL;Ginsburg;J. Assoc. Comput. Mach.,1962