@article{TAC_2012_26_a25,
     author = {Susan Niefield},
     title = {Span, cospan, and other double categories},
     journal = {Theory and applications of categories},
     pages = {729--742},
     publisher = {mathdoc},
     volume = {26},
     year = {2012},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TAC_2012_26_a25/}
}