Evolve only the exterior of the hole, ignore interior.
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No boundary condition should be imposed at the horizon.
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Coordinates are well-behaved:
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Coordinates do not fall into a stationary hole
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Hole is allowed to move through the grid.
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Requires large shift vector: difficult in computations.
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Using CD, quantities are evolved along the center of the local light cone, then placed at their spatial locations.
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Casual differencing allows for arbitrary choice of shift vector, makes the courant condition independent of the shift, naturally handles black hole excision, moving holes.
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