@article{CMH_1982__57_139923,
     author = {Irwin Kra and Bernard Maskit},
     title = {Bases for qualitativ differentials.},
     journal = {Commentarii mathematici Helvetici},
     pages = {603--626},
     publisher = {mathdoc},
     volume = {57},
     year = {1982},
     zbl = {0527.30036},
     url = {http://geodesic.mathdoc.fr/item/CMH_1982__57_139923/}
}