@article{SF_1974__9_134066,
     author = {P.J. Trotter},
     title = {Congruence-free inverse semigroups.},
     journal = {Semigroup forum},
     pages = {109--116},
     publisher = {mathdoc},
     volume = {9},
     year = {1974},
     zbl = {0293.20053},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__9_134066/}
}