@article{JRAM_1991__417_153329,
     author = {Alan Weinstein and Ping Xu},
     title = {Extensions of symplectic groupoids and quantization.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {159--190},
     publisher = {mathdoc},
     volume = {417},
     year = {1991},
     zbl = {0722.58021},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1991__417_153329/}
}