@article{SF_1974__6_133981,
     author = {J. Shafer},
     title = {Homomorphisms of free contents.},
     journal = {Semigroup forum},
     pages = {255--259},
     publisher = {mathdoc},
     volume = {6},
     year = {1974},
     zbl = {0272.20061},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__6_133981/}
}