@article{10_4153_CJM_1973_005_8,
     author = {Osterweil, Leon},
     title = {A {Theorem} for {Enumerating} {Certain} {Types} of {Collections}},
     journal = {Canadian journal of mathematics},
     pages = {74--82},
     year = {1973},
     volume = {25},
     number = {1},
     doi = {10.4153/CJM-1973-005-8},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1973-005-8/}
}