@article{SF_1991__43_3_135106,
     author = {C.P. Rupert},
     title = {On commutative {Kleene} monoids.},
     journal = {Semigroup forum},
     pages = {163--177},
     year = {1991},
     volume = {43},
     number = {3},
     zbl = {0762.68048},
     url = {http://geodesic.mathdoc.fr/item/SF_1991__43_3_135106/}
}