@article{ARM_1973_9_1_a2,
     author = {Gruska, Jozef},
     title = {Grammatical levels and subgrammars of context-free grammars},
     journal = {Archivum mathematicum},
     pages = {19--21},
     year = {1973},
     volume = {9},
     number = {1},
     mrnumber = {0351163},
     zbl = {0282.68036},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ARM_1973_9_1_a2/}
}