@article{AMLG_1975__17_137899,
     author = {Ch.E. Hughes and W.E. Singletary},
     title = {Combinatorial systems defined over one- and two-letter alphabets.},
     journal = {Archiv f\"ur mathematische Logik und Grundlagenforschung},
     pages = {25--34},
     publisher = {mathdoc},
     volume = {17},
     year = {1975},
     zbl = {0336.02035},
     url = {http://geodesic.mathdoc.fr/item/AMLG_1975__17_137899/}
}