If the
In a BUG, each value that remains in a row, column or block has exactly two possible positions in that row, column or block; and each empty cell has exactly two possible values. A Sudoku having a BUG has zero, two or more valid solutions.
Because a valid Sudoku has exactly one solution, the BUG cannot be part of it.
The only way to avoid the BUG is if at least one of the