Sue de Coq
An advanced intersection technique that combines naked and hidden subsets across a box and one of its lines at the same time.
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
- 1A1 and B1 are the intersection — the cells the top-left box and row 1 share — and between two cells they hold four digits, {1, 2, 3, 4}.
- 2A3 covers two of them from inside the box ({1, 2}) and F1 covers the other two from along the row ({3, 4}). Those four cells see each other enough that their four values must be exactly 1, 2, 3 and 4.
- 3Neither 1 nor 2 can end up at F1, so both live in A1, B1 or A3 — all inside the box. Strike 1 from C2, and any other box cell holding 1 or 2.
- 4By the same split, 3 and 4 can't be at A3, so they live in A1, B1 or F1 — all along row 1. Strike 3 from D1. Neither elimination needs you to know which intersection cell holds what.
- First colour in the chain
- Second colour in the chain
- Cells that establish the pattern
- Where the conclusion lands
- Candidate ruled out