@article{SF_1974__6_133960,
     author = {R.A.R. Monzo},
     title = {Categorical semigroups.},
     journal = {Semigroup forum},
     pages = {59--68},
     publisher = {mathdoc},
     volume = {6},
     year = {1974},
     zbl = {0255.20037},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__6_133960/}
}