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Naked Subsets

  If X number of cells in a row, column or box contain X number of different candidates, then those candidates can be eliminated from all other cells in that unit. Consider the example on this page. In the bottom row, the three highlighted cells only contain the candidates 2, 5 and 9. That's three cells with three candidates - a naked subset. Thus, all other 2's, 5's and 9's in the bottom row can be eliminated. This is because one of the three highlighted cells must be a 2, one must be a 5, and one must be a 9. Similarly if two cells have only two candidates, then those two candidates can be eliminated everywhere else in the unit.

  Wherever there is a naked subset, by logic there must also be a hidden subset. Thus, if the naked subset has only two or three cells, the naked subset takes precedence in the rating system. On the rare occasion that a unit with eight or nine open cells contains a naked/hidden subset of four cells, the naked subset takes precedence once again.
  Sudoku Snake gives naked subsets a skill value of 35.

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