@article{FPM_1997_3_4_a17,
     author = {T. L. Sidon},
     title = {Provability logic with operations over proofs},
     journal = {Fundamentalʹna\^a i prikladna\^a matematika},
     pages = {1173--1197},
     year = {1997},
     volume = {3},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/FPM_1997_3_4_a17/}
}