@article{PIM_1980_N_S_27_41_a29,
     author = {Marica D. Pre\v{s}i\'c},
     title = {On {Free} {Commutative} {Groupoids}},
     journal = {Publications de l'Institut Math\'ematique},
     pages = {209 },
     publisher = {mathdoc},
     volume = {_N_S_27},
     number = {41},
     year = {1980},
     url = {http://geodesic.mathdoc.fr/item/PIM_1980_N_S_27_41_a29/}
}