@article{SF_1994__49_2_135358,
     author = {R. Broeksteeg},
     title = {A concept of variety for regular biordered sets.},
     journal = {Semigroup forum},
     pages = {335--348},
     publisher = {mathdoc},
     volume = {49},
     number = {2},
     year = {1994},
     zbl = {0814.20041},
     url = {http://geodesic.mathdoc.fr/item/SF_1994__49_2_135358/}
}