@article{10_4153_CJM_1979_003_8,
     author = {Rival, Ivan and Sands, Bill},
     title = {Planar {Sublattices} of a {Free} {Lattice.} {II}},
     journal = {Canadian journal of mathematics},
     pages = {17--34},
     year = {1979},
     volume = {31},
     number = {1},
     doi = {10.4153/CJM-1979-003-8},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1979-003-8/}
}