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Finned Swordfish

  A Finned Swordfish is a pattern that is one cell short of being a swordfish - in other words, three columns that contain a candidate in only three rows, except for one, and vice versa. The logic behind this technique is that if the fin is not true, the rest of the Swordfish is.
  Consider the example on this page. Rows 2, 6 and 9 from the top contain the candidate 3 only in columns 2, 5 and 8, except for the stray 3 in column 7, row 2. If the cell in column 7, row 2 is NOT a 3, then the swordfish pattern is true and you can eliminate all the other 3's in columns 2, 5 and 8. However, if the cell in column 7, row 2 IS a 3, you can eliminate all other 3's in the upper-right box. Either way, the 3 highlighted in green 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 9, row 2 also contained the candidate 3, 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 Swordfish 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 Swordfish a skill value of 250.

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