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We are looking for three cells in two groups (row, column or 3x3 box)
that each contain exactly two candidates and they shares the same three candidates.
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In the example above, a xy-wing is formed by the cells R2C2,
R2C5 and R8C5.
(R2C2 and R2C5 are in the same row,
and R2C5 and R8C5 are in the same column.)
Since either R2C2 or R8C5 must contain 2,
candidate 2 in R8C2 (highlighted yellow)
can safely be removed.
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