@article{KYB_1967__3_4_a0,
     author = {Kr\'al, Jaroslav},
     title = {Set-theoretical operations on $k$-multiple languages},
     journal = {Kybernetika},
     pages = {--320},
     publisher = {mathdoc},
     volume = {03},
     number = {4},
     year = {1967},
     zbl = {0155.01604},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/KYB_1967__3_4_a0/}
}