@article{KYB_1967_3_1_a1,
     author = {Gruska, Jozef},
     title = {On a classification of context-free languages},
     journal = {Kybernetika},
     pages = {22--29},
     year = {1967},
     volume = {3},
     number = {1},
     mrnumber = {0209092},
     zbl = {0158.25401},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/KYB_1967_3_1_a1/}
}