@article{CTGDC_1972__13_4_357_0, author = {Barr, Michael}, title = {The point of the empty set}, journal = {Cahiers de topologie et g\'eom\'etrie diff\'erentielle}, pages = {357--368}, publisher = {Dunod \'editeur, publi\'e avec le concours du CNRS}, volume = {13}, number = {4}, year = {1972}, zbl = {0258.18003}, mrnumber = {323861}, language = {en}, url = {http://www.numdam.org/item/CTGDC_1972__13_4_357_0/} }