[1] Ginsburg S.: The mathematical theory of context-free languages. McGraw-Hill, New York 1966. MR 0211815 | Zbl 0184.28401

[2] Gruska J.: On a classification of context-free grammars. Kybernetika 3 (1967), 1, 22-29. MR 0209092

[3] Gruska J.: Some classifications of context-free languages. Information and Control 14 (1969), 152-179. MR 0253836 | Zbl 0174.28901

[4] Gruska J.: Complexity and unambiguity of context-free grammars and languages. Information and Control 18 (1971), 502-519. MR 0311152 | Zbl 0238.68022

[5] Taniguchi K., Kasami T.: Reduction of Context-Free Grammars. Information and Control 17 (1970), 92-108. MR 0272574 | Zbl 0214.02005