@article{SF_1974__6_133972,
     author = {J.C. Spehner},
     title = {Demi-groupes prelibres.},
     journal = {Semigroup forum},
     pages = {171--179},
     publisher = {mathdoc},
     volume = {6},
     year = {1974},
     zbl = {0255.20042},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__6_133972/}
}