@article{DAN_1961_139_2_a12,
     author = {Ya. S. Smetanich},
     title = {On statement calculi with an additional operation},
     journal = {Doklady Akademii Nauk},
     pages = {309--312},
     year = {1961},
     volume = {139},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1961_139_2_a12/}
}