  The force-free limit of magnetohydrodynamics (MHD) is often a reasonable
approximation to model black hole and neutron star magnetospheres. We describe
a general relativistic force-free (GRFFE) formulation that allows general
relativistic magnetohydrodynamic (GRMHD) codes to directly evolve the GRFFE
equations of motion. Established, accurate, and well-tested conservative GRMHD
codes can simply add a new inversion piece of code to their existing code,
while continuing to use all the already-developed facilities present in their
GRMHD code. We show how to enforce the $\mathbf{E}\cdot\mathbf{B}=0$ constraint
and energy conservation, and we introduce a simplified general model of the
dissipation of the electric field to enforce the $B^2-E^2>0$ constraint. We
also introduce a simplified yet general method to resolve current sheets,
without much reconnection, over many dynamical times. This formulation is
incorporated into an existing GRMHD code (HARM), which is demonstrated to give
accurate and robust GRFFE results for Minkowski and black hole space-times.
