Crowding a Clue
A bulb that would blind more of a clue's remaining neighbours than the clue can spare is impossible.
How it works
- 1This wall wants three bulbs from the four cells touching it, so it can afford to lose exactly one of them.
- 2A bulb on the diagonal cell would shine along its row and down its column, and hit two of those four at once.
- 3That would leave only two places for three bulbs, which is impossible.
- 4So the diagonal cell is empty. The rule generalises: count how many of a clue's free neighbours a candidate would blind, and rule it out whenever that exceeds the clue's slack.
- Cells that establish the pattern
- Cell this technique rules out