@article{SF_1972__5_133944,
     author = {W.D. Munn},
     title = {Free inverse semigroups.},
     journal = {Semigroup forum},
     pages = {262--269},
     publisher = {mathdoc},
     volume = {5},
     year = {1972},
     zbl = {0274.20076},
     url = {http://geodesic.mathdoc.fr/item/SF_1972__5_133944/}
}