@article{SE_1960-1962__3__A5_0,
     author = {Ehresmann, Charles},
     title = {\'Elargissements de cat\'egories},
     journal = {S\'eminaire Ehresmann. Topologie et g\'eom\'etrie diff\'erentielle},
     note = {talk:4},
     publisher = {Secr\'etariat math\'ematique},
     volume = {3},
     year = {1960-1962},
     zbl = {0106.36302},
     language = {fr},
     url = {archive.numdam.org/item/SE_1960-1962__3__A5_0/}
}

Ehresmann, C. Élargissements de catégories. Séminaire Ehresmann. Topologie et géométrie différentielle, Tome 3 (1960-1962) , Exposé no. 4, 49 p. http://archive.numdam.org/item/SE_1960-1962__3__A5_0/