Linked Sources
A numbered clue and a dark cell's possible light sources can constrain one another just like a pair of clues.
How it works
- 1The crossed dark cell at row 3 column 2 has only two possible light sources left: row 1 column 2 or row 3 column 1.
- 2The corner 1 likewise needs exactly one of row 1 column 2 and row 2 column 1.
- 3Row 2 column 1 sees row 3 column 1, so those two cannot both be bulbs. If row 1 column 2 were empty, both obligations would force that forbidden pair.
- 4Therefore row 1 column 2 is a bulb. This is neither two clues nor coverage alone: the conclusion appears only after linking one constraint of each kind.
- Cells that establish the pattern
- Bulb this technique places