BUG+1

A worked example

This position comes from SudokuByte’s lesson on the BUG+1. Gold cells hold the evidence; green cells are the pattern’s ends.

1. Where to look

   Every empty cell but one is bivalue. Removing the exceptional candidate makes each remaining digit occur zero or twice in every house.

   A nearly binary grid has one extra candidate that must prevent an ambiguous binary pattern. The exceptional cell takes that candidate.

2. Read the evidence in this position

   Every empty cell except r9c7 is bivalue. That exceptional cell has 2, 5, 7.

3. Why the deduction must follow

   Remove 7 from that cell just for the test: now every unsolved digit occurs exactly twice in each relevant row, column, and box, and every empty cell is bivalue. The binary ambiguity conflicts with a unique solution. So the exceptional candidate 7 must be true. Merely having one trivalue cell would not have been enough.

4. Apply the result

   Place 7 in r9c7.