Variable displacement

When $b=1$, the center column must be periodic. This means that all cells not dependent on $b$ must be periodic regardless of anything else.

When $b=0$, then $a$ controls whether the left or center column behaves as the typical center column. If $a=0$, then the center is the center and must be periodic; if $a=1$, then the left column becomes the center and must be periodic.

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