@article{CMUC_1967_8_1_a11,
     author = {Vop\v{e}nka, Petr},
     title = {General theory of $\nabla$-models},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {145--170},
     year = {1967},
     volume = {8},
     number = {1},
     mrnumber = {0214460},
     zbl = {0162.01701},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMUC_1967_8_1_a11/}
}