@article{SF_1979__18_134344,
     author = {A.H. Clifford},
     title = {The free completely regular semigroup on a set.},
     journal = {Semigroup forum},
     pages = {87--92},
     publisher = {mathdoc},
     volume = {18},
     year = {1979},
     zbl = {0462.20047},
     url = {http://geodesic.mathdoc.fr/item/SF_1979__18_134344/}
}