Bivalue Universal Grave type 3 (with Naked {4})

If the values {0} were removed from the cells {1}, the Sudoku would exhibit a Bivalue Universal Grave pattern (or BUG).

In a BUG, each value that remains in a row, column or box has exactly two possible positions in that row, column or box; and each empty cell has exactly two potential 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 cells {2} contains one of the values {3}. It follows that one of {2} forms a Naked {4} with {5} on the values {6} in the {7}.

Other possible positions of the values {6} can therefore be removed from the {7}.