@article{SF_1993__46_1_135212,
     author = {S. Schanuel and J. Isbell},
     title = {Some closure properties in monoids.},
     journal = {Semigroup forum},
     pages = {73--77},
     publisher = {mathdoc},
     volume = {46},
     number = {1},
     year = {1993},
     zbl = {0776.20022},
     url = {http://geodesic.mathdoc.fr/item/SF_1993__46_1_135212/}
}