@article{SF_1974__9_134059,
     author = {Kenneth E. Oberhoff},
     title = {Semilattice structures on trees.},
     journal = {Semigroup forum},
     pages = {54--73},
     publisher = {mathdoc},
     volume = {9},
     year = {1974},
     zbl = {0306.54042},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__9_134059/}
}