Cactus Code Thorn CarpetMask
Thorn Author(s)     : Erik Schnetter <schnetter@cct.lsu.edu>
Thorn Maintainer(s) : Erik Schnetter <schnetter@cct.lsu.edu>
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Purpose of the thorn:

Remove unwanted regions from Carpet's reduction operations.  This can
be used e.g. to excise the interior of horizons.