@article{AM2_1980__21_182432,
     author = {M.J. Pelling and D.G. Rogers},
     title = {Some (2, n) combinatorial {Stein} groupoids.},
     journal = {Aequationes mathematicae},
     pages = {225--235},
     year = {1980},
     volume = {21},
     zbl = {0465.20064},
     url = {http://geodesic.mathdoc.fr/item/AM2_1980__21_182432/}
}