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@article{KYB_1967__3_1_a1, author = {Gruska, Jozef}, title = {On a classification of context-free languages}, journal = {Kybernetika}, pages = {--29}, publisher = {mathdoc}, volume = {03}, number = {1}, year = {1967}, mrnumber = {0209092}, zbl = {0158.25401}, language = {en}, url = {http://geodesic.mathdoc.fr/item/KYB_1967__3_1_a1/} }
Gruska, Jozef. On a classification of context-free languages. Kybernetika, Tome 03 (1967) no. 1, p. 29. http://geodesic.mathdoc.fr/item/KYB_1967__3_1_a1/