@article{SF_1993__46_1_135227,
     author = {V. Koubek},
     title = {Universal expansion of semigroup varieties by regular involution.},
     journal = {Semigroup forum},
     pages = {152--159},
     year = {1993},
     volume = {46},
     number = {1},
     zbl = {0780.20038},
     url = {http://geodesic.mathdoc.fr/item/SF_1993__46_1_135227/}
}