Cactus Code Thorn Boundary
Author(s)    : Gabrielle Allen
               Gerd Lanfermann
               Miguel Alcubierre
               David Rideout
Maintainer(s): Cactus team
Licence      : LGPL
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1. Purpose

This thorn defines a generic interface for boundary conditions, which
allows, among other things, symmetry boundary conditions associated
with the grid to be applied without other thorns having knowledge of
them.

This thorn also provides some standard outer boundary conditions.

Currently:

  - scalar boundary conditions

  - flat boundary conditions (a copy of the point just inside the
    boundary)
 
  - radiation boundary conditions

  - copy boundary conditions 

  - Robin boundary conditions

  - static boundary conditions


2. Additional information

Each of the boundary conditions mentioned above work only on grid
variables with less than four dimensions.

All the boundary conditions work with an arbitrarily size stencil
width, except for the Robin boundary condition which currently only
works with a stencil width of one.

The Robin boundary condition is only implemented for 3D grid functions
at the moment.