@article{1183732304,
author = {Copi, Irving M.},
title = {Another Variant of Natural Deduction},
journal = {J. Symbolic Logic},
volume = {21},
number = {1},
year = {1956},
pages = { 52-55},
language = {en},
url = {http://dml.mathdoc.fr/item/1183732304}
}
Copi, Irving M. Another Variant of Natural Deduction. J. Symbolic Logic, Tome 21 (1956) no. 1, pp. 52-55. http://gdmltest.u-ga.fr/item/1183732304/