  Three-dimensional excitable systems can selforganize vortex patterns that
rotate around one-dimensional phase singularities called filaments. In
experiments with the Belousov-Zhabotinsky reaction and numerical simulations,
we pin these scroll waves to moving heterogeneities and demonstrate the
controlled repositioning of their rotation centers. If the pinning site extends
only along a portion of the filament, the phase singularity is stretched out
along the trajectory of the heterogeneity which effectively writes the
singularity into the system. Its trailing end point follows the heterogeneity
with a lower velocity. This velocity, its dependence on the placement of the
anchor, and the shape of the filament are explained by a curvature flow model.
