@article{SF_1980__21_134438,
     author = {J. Anderson},
     title = {Closure operators on languages and binary relations.},
     journal = {Semigroup forum},
     pages = {91--92},
     publisher = {mathdoc},
     volume = {21},
     year = {1980},
     zbl = {0451.20059},
     url = {http://geodesic.mathdoc.fr/item/SF_1980__21_134438/}
}