@article{SF_1974__6_133980,
     author = {A.H. Clifford and T.E. Hall},
     title = {A characterization of ...-classes of semigroups as partial groupoids.},
     journal = {Semigroup forum},
     pages = {246--254},
     publisher = {mathdoc},
     volume = {6},
     year = {1974},
     zbl = {0262.20067},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__6_133980/}
}