Takustraße 7 D-14195 Berlin-Dahlem Germany Konrad-Zuse-Zentrum f¨ ur Informationstechnik Berlin E RIC BARTH,B ENEDICT L EIMKUHLER,S EBASTIAN R EICH A Time-Reversible Variable-Stepsize Integrator for Constrained Dynamics Preprint SC 97-53 (December 1997)