An X-wing is found when:
1). A candidate is present exactly twice in a row (the 4 in row 2 in this example)
2). It is also present exactly twice in another row (row 8) and in the same two columns (c3 and c6)
When this happens, that candidate can be eliminated from the remaining cells
in those two columns.
(X-wings can be found in columns too eliminating candidates in rows)
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