@article{PIM_1972_N_S_14_28_a6,
     author = {Milan {\DJ}uri\'c},
     title = {Some {Fundamental} {Structures} {On} {Classes}},
     journal = {Publications de l'Institut Math\'ematique},
     pages = {49 },
     publisher = {mathdoc},
     volume = {_N_S_14},
     number = {28},
     year = {1972},
     url = {http://geodesic.mathdoc.fr/item/PIM_1972_N_S_14_28_a6/}
}