Bi-value Universal Grave
A Bi-value Universal Grave (BUG) is a grid state where every unsolved cell contains exactly two candidates, and no Hidden Singles are present. This is a type of Deadly Pattern that cannot appear in a valid Sudoku puzzle
- We should avoid this deadly pattern from appearing in our Sudoku. Consider this example
- All of these cells, except the highlighted one, have exactly two candidates left, while the highlighted cell has three candidates
- If we were to remove candidate 3 from this cell, we would get more than one valid solution for this Sudoku. So we can safely remove all other candidates from this cell
