@article{SF_1970__1_133801,
     author = {A.H. Clifford},
     title = {A proof of the {Montague-Plemmons-Schein} {Theorem} on maximal subgroups of the semigroup of binary relations.},
     journal = {Semigroup forum},
     pages = {272--275},
     publisher = {mathdoc},
     volume = {1},
     year = {1970},
     zbl = {0211.03902},
     url = {http://geodesic.mathdoc.fr/item/SF_1970__1_133801/}
}