@article{SF_1976__13_134202,
     author = {J.E. McMorris},
     title = {The topological structure of planar closed spirals which support clans.},
     journal = {Semigroup forum},
     pages = {115--118},
     year = {1976},
     volume = {13},
     zbl = {0362.22008},
     url = {http://geodesic.mathdoc.fr/item/SF_1976__13_134202/}
}