@article{SF_1976__13_134228,
     author = {T. E. Hays},
     title = {Trees and monotone structures.},
     journal = {Semigroup forum},
     pages = {377--384},
     year = {1976},
     volume = {13},
     zbl = {0356.22001},
     url = {http://geodesic.mathdoc.fr/item/SF_1976__13_134228/}
}