@article{MZ_1988__198_3_173908,
     author = {A.W. Mason},
     title = {Free {Quotients} of {Congruence} {Subgroups} of {SL2} {Over} a {Coordinate} {Ring.}},
     journal = {Mathematische Zeitschrift},
     pages = {39--52},
     publisher = {mathdoc},
     volume = {198},
     number = {3},
     year = {1988},
     zbl = {0619.20031},
     url = {http://geodesic.mathdoc.fr/item/MZ_1988__198_3_173908/}
}