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Finned X Wings |
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A Finned X-Wing is a pattern that is one cell short of being an X-Wing - in other words, two columns that contain a candidate in only two rows, except for one (the fin), and vice versa. The logic behind this technique is that if the fin is not true, the rest of the X-Wing is.
Consider the example on this page. Columns 1 and 6 from the left only have 7's in rows 2 and 4 from the top, except for the one in column 6, row 5. If column 6, row 5 is NOT a 7, then we have an X-Wing and can eliminate all other 7's in rows 2 and 4. However, if column 6, row 5 IS a 7, we can eliminate all other 7's in the middle Box. Either way, the 7 in column 5, row 4 can be eliminated.
Not all fins must be only one cell. Sometimes a fin can extend to several cells that share the same tertiary unit. For instance, if in the example on this page the cell in column 6, row 6 (which here contains a 2) was empty and contained the candidate 7, it would still be an extension of the same fin. The logic would then be as follows. If neither of the cells in the fin is true, the X-Wing is true. Otherwise, if either one of the fin cells is true, we can still make the elimination hightlighted in green.
Sudoku Snake gives Finned X-Wings a skill value of 90.
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