@article{TAC_2010_23_a10,
     author = {Zurab Janelidze},
     title = {The pointed subobject functor, 3 x 3 lemmas, and subtractivity of spans},
     journal = {Theory and applications of categories},
     pages = {221--242},
     year = {2010},
     volume = {23},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TAC_2010_23_a10/}
}