@article{JRAM_1977__295_151924,
     author = {Siegfried Graf},
     title = {A selection theorem for {Boolean} correspondences.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {169--186},
     year = {1977},
     volume = {295},
     zbl = {0366.06022},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1977__295_151924/}
}