Pattern Overlay Method
Every way a single digit could legally fill the whole board is enumerated as a "pattern"; a candidate present in none of the surviving patterns can be eliminated.
The solver does not attempt this technique, so Smart Hint will never suggest it. It is documented because you may still want to spot it yourself.
How it works
- 1Enumerate every valid way digit 5 alone could be placed once in each row, column, and box across the whole grid, given the current candidates — each valid arrangement is one "pattern."
- 2If A1 = 5 doesn't appear in any surviving pattern for digit 5, then 5 can be eliminated from A1 — even though no shorter chain of deductions reaches that conclusion directly. Exhaustive and computationally heavy; a genuine last-resort-adjacent technique.
- Where the conclusion lands
- Candidate ruled out