@article{10_21136_CMJ_1975_101352,
     author = {Havel, V\'aclav},
     title = {Universality property of free groupoid extensions of halfgroupoids and its geometrical meaning},
     journal = {Czechoslovak Mathematical Journal},
     pages = {562--567},
     publisher = {mathdoc},
     volume = {25},
     number = {4},
     year = {1975},
     doi = {10.21136/CMJ.1975.101352},
     mrnumber = {0412304},
     zbl = {0327.20022},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.21136/CMJ.1975.101352/}
}