@article{ZNSL_1967_4_a7,
     author = {V. A. Lifshits},
     title = {Deductive validity and reduction classes},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {69--77},
     year = {1967},
     volume = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1967_4_a7/}
}