@article{PIM_1977_N_S_22_36_a33,
     author = {Lee Sin-Min},
     title = {Permutation {Group} {Extension} {Of} {Groupoids}},
     journal = {Publications de l'Institut Math\'ematique},
     pages = {255 },
     publisher = {mathdoc},
     volume = {_N_S_22},
     number = {36},
     year = {1977},
     url = {http://geodesic.mathdoc.fr/item/PIM_1977_N_S_22_36_a33/}
}