@article{GDZPPN00247607X, title = {An Elementary Proof of a Theorem of MacLane.}, journal = {Monatshefte f\"ur Mathematik}, volume = {72}, year = {1968}, pages = {144-146}, zbl = {0165.40202}, url = {http://dml.mathdoc.fr/item/GDZPPN00247607X} }
Schneider, W.J. An Elementary Proof of a Theorem of MacLane.. Monatshefte für Mathematik, Tome 72 (1968) pp. 144-146. http://gdmltest.u-ga.fr/item/GDZPPN00247607X/