@article{SF_1989__38_2_134942,
     author = {Edward J. Tully},
     title = {Expletives in languages and middle units in semigroups.},
     journal = {Semigroup forum},
     pages = {77--84},
     publisher = {mathdoc},
     volume = {38},
     number = {2},
     year = {1989},
     zbl = {0662.20043},
     url = {http://geodesic.mathdoc.fr/item/SF_1989__38_2_134942/}
}