@article{MZ_1962__80_170093,
     author = {Johann Sonner},
     title = {On the formal definition of categories.},
     journal = {Mathematische Zeitschrift},
     pages = {163--176},
     publisher = {mathdoc},
     volume = {80},
     year = {1962},
     zbl = {0109.24202},
     url = {http://geodesic.mathdoc.fr/item/MZ_1962__80_170093/}
}