@article{10_1017_S0017089500007394,
     author = {Hofmann, K. H. and Jr, K. D. Magill},
     title = {The smallest proper congruence on {S(X)}},
     journal = {Glasgow mathematical journal},
     pages = {301--313},
     year = {1988},
     volume = {30},
     number = {3},
     doi = {10.1017/S0017089500007394},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500007394/}
}