@article{SF_1974__9_134081,
     author = {Ki-Hang, Kim Butler},
     title = {The semigroup of {Hall} relations.},
     journal = {Semigroup forum},
     pages = {253--260},
     publisher = {mathdoc},
     volume = {9},
     year = {1974},
     zbl = {0292.20061},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__9_134081/}
}