@article{MM2_1991__73_1_155674,
     author = {B. Baumann},
     title = {Free pro-p groups with operators.},
     journal = {Manuscripta mathematica},
     pages = {385--396},
     publisher = {mathdoc},
     volume = {73},
     number = {1},
     year = {1991},
     zbl = {0755.20006},
     url = {http://geodesic.mathdoc.fr/item/MM2_1991__73_1_155674/}
}