@article{PIM_1993_N_S_54_68_a18,
     author = {Gordana Pavlovi\'c-La\v{z}eti\'c},
     title = {Inference in a {Relation} {Knowledge-base} {With} two {Types} of {Null} {Values}},
     journal = {Publications de l'Institut Math\'ematique},
     pages = {156 },
     publisher = {mathdoc},
     volume = {_N_S_54},
     number = {68},
     year = {1993},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PIM_1993_N_S_54_68_a18/}
}