BUG +1 (Bivalue Universal Grave)
When every empty cell except one has exactly two candidates, the single trivalue cell's odd-digit-out is the solution.
Advanced sudoku technique
What it is
A 'Bivalue Universal Grave' is a deadly pattern: a grid state where every empty cell has exactly two candidates, every digit has exactly two candidate cells in each row/column/box. Such a state has TWO valid completions by symmetry, so a valid sudoku can never reach it. BUG +1 is the practical case: the grid is one step away from a full BUG — every empty cell has exactly two candidates EXCEPT for one outlier cell with three candidates. The outlier's third candidate (the one that appears three times in some unit instead of two) is what breaks the deadly symmetry — and since the puzzle has a unique solution, that third candidate is the answer.
When to use it
On the deepest expert and master puzzles. The signal: late in the solve, the candidate map shows every empty cell as bivalue except one trivalue outlier.
Worked example
Late in a hard solve, every empty cell has exactly two candidates EXCEPT R3C5 = {1, 4, 7}. Inspect row 3: digit 4 appears in three cells in row 3, while digits 1 and 7 each appear in two. The 'extra' digit is 4 — R3C5 = 4 breaks the deadly pattern.
Try it
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