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This solving technique is based
on avoiding a deadly rectangle.
A deadly rectangle can only exist
in an invalid puzzle (multiple solutions).
The four yellow squares shown are close to becoming a deadly
rectangle. If the cell in r9c8 was a 1 or a 6, then
the deadly rectangle would exist. Since it is known that this is
a valid puzzle, a deadly rectangle is not possible.
Therefore the 1 and 6 can be eliminated from r9c8.
(back)
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