@article{SF_1991__43_3_135118,
     author = {J. Kadourek},
     title = {On varieties of combinatorial inverse semigroups {I.}},
     journal = {Semigroup forum},
     pages = {305--330},
     year = {1991},
     volume = {43},
     number = {3},
     zbl = {0743.20057},
     url = {http://geodesic.mathdoc.fr/item/SF_1991__43_3_135118/}
}