@article{SF_1978__16_134271,
     author = {C. Wells},
     title = {Extension theories for monoids.},
     journal = {Semigroup forum},
     pages = {13--36},
     publisher = {mathdoc},
     volume = {16},
     year = {1978},
     zbl = {0394.18007},
     url = {http://geodesic.mathdoc.fr/item/SF_1978__16_134271/}
}