Cactus Code Thorn CarpetMask
Author(s)    : Erik Schnetter <schnetter@cct.lsu.edu>
Maintainer(s): Erik Schnetter <schnetter@cct.lsu.edu>
Licence      : GPLv2+
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1. Purpose

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