
Hidden Subsets 

If X number of candidates in a row, column or box are only found in X number of cells, then all other candidates in those cells can be eliminated. Consider the example on this page. In the upperright box, the candidates 2 and 8 are only found in the two highlighted cells. That's two candidates only in two cells  a hidden subset. Thus, all other candidates in those cells can be eliminated. This is because one cell must be a 2 and the other must be an 8. Similarly if three candidates are only found in three cells, all other candidates in those cells can be eliminated.
Wherever there is a hidden subset, by logic there must also be a naked subset. Thus, if the naked subset has two or three cells, the naked subset takes precedence in the rating system over the hidden subset. 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 hidden subsets a skill value of 55.






