extend.c
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If no filename is given, a default magnetization will be created: All vertices lying above the plane through parallel to the x-y-plane will be assigned the magnetization vector (-1, 0, 0). All vertices lying in or below that plane will be assigned a magnetization vector (+1, 0, 0). Thus two domains are created. The upper one is magnetized in negative x-direction, the lower in positive x-direction.