@article{10_21136_CMJ_1967_100776,
     author = {\v{C}ul{\'\i}k, Karel},
     title = {On some transformations in context-free grammars and languages},
     journal = {Czechoslovak Mathematical Journal},
     pages = {278--311},
     publisher = {mathdoc},
     volume = {17},
     number = {2},
     year = {1967},
     doi = {10.21136/CMJ.1967.100776},
     mrnumber = {0214410},
     zbl = {0204.02103},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.21136/CMJ.1967.100776/}
}