Light Up techniques

A cell that is already lit can never hold a bulb — whatever lights it would be looking straight at it.

A clue with all the bulbs it asked for is finished: every other cell touching it is empty.

When a clue has exactly as many free neighbours left as bulbs still to place, all of them are bulbs.

An unlit cell with only one cell left that could light it: that cell holds a bulb.

A bulb that would blind more of a clue's remaining neighbours than the clue can spare is impossible.

A bulb that would blind every remaining source of some unlit cell is impossible.

Two clues that share neighbours, or whose neighbours see each other, often leave only one arrangement they both allow.

A numbered clue and a dark cell's possible light sources can constrain one another just like a pair of clues.

Try a cell both ways: reject a branch that fails, or keep any other mark both surviving branches agree on.

Easy