@article{SM_1991_69_1_a14,
     author = {V. Yu. Shavrukov},
     title = {On two extensions of the logic of {provability~GL}},
     journal = {Sbornik. Mathematics},
     pages = {255--270},
     publisher = {mathdoc},
     volume = {69},
     number = {1},
     year = {1991},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1991_69_1_a14/}
}