Gerald J. Massey. Note on Copi's system. Notre Dame Journal of Formal Logic, 4(2):140-141, 1963. [doi]
@article{Massey63, title = {Note on Copi's system}, author = {Gerald J. Massey}, year = {1963}, doi = {10.1305/ndjfl/1093957504}, url = {http://dx.doi.org/10.1305/ndjfl/1093957504}, researchr = {https://researchr.org/publication/Massey63}, cites = {0}, citedby = {0}, journal = {Notre Dame Journal of Formal Logic}, volume = {4}, number = {2}, pages = {140-141}, }