@article{CMUC_1973_14_3_a1,
     author = {H\'ajek, Petr},
     title = {Why semisets?},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {397--420},
     year = {1973},
     volume = {14},
     number = {3},
     mrnumber = {0342393},
     zbl = {0268.02005},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMUC_1973_14_3_a1/}
}