@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},
     mrnumber = {323861},
     zbl = {0258.18003},
     language = {en},
     url = {http://archive.numdam.org/item/CTGDC_1972__13_4_357_0/}
}