@article{10_4153_CJM_1970_009_7,
     author = {Hilton, P. J. and Wu, Y.-C.},
     title = {On the {Addition} of {Relations} in an {Abelian} {Category}},
     journal = {Canadian journal of mathematics},
     pages = {66--74},
     year = {1970},
     volume = {22},
     number = {1},
     doi = {10.4153/CJM-1970-009-7},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1970-009-7/}
}