@article{1093890618,
author = {vander Nat, Arnold},
title = {Axiomatic, sequenzen-kalkul, and subordinate proof versions of ${\rm S9}$.},
journal = {Notre Dame J. Formal Logic},
volume = {13},
number = {3},
year = {1972},
pages = { 309-322},
language = {en},
url = {http://dml.mathdoc.fr/item/1093890618}
}
vander Nat, Arnold. Axiomatic, sequenzen-kalkul, and subordinate proof versions of ${\rm S9}$.. Notre Dame J. Formal Logic, Tome 13 (1972) no. 3, pp. 309-322. http://gdmltest.u-ga.fr/item/1093890618/