@article{AL_1984_23_3_a7,
     author = {A. A. Shum},
     title = {Varieties of algebraic systems and propositional calculi},
     journal = {Algebra i logika},
     pages = {341--359},
     publisher = {mathdoc},
     volume = {23},
     number = {3},
     year = {1984},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1984_23_3_a7/}
}