@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/