@article{TAC_2010_24_a12,
     author = {Robert Dawson and Robert Par\'e and Dorette Pronk},
     title = {The span construction},
     journal = {Theory and applications of categories},
     pages = {302--377},
     publisher = {mathdoc},
     volume = {24},
     year = {2010},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TAC_2010_24_a12/}
}