Sentinels #82

• Last cell in a region. Every coloured region holds exactly one sentinel. A region down to a single open cell is solved.
• Line locked to a region. The mirror image: if every open cell of a row lies inside a single region, that region’s sentinel is that row’s sentinel, so the region’s cells outside the row can be crossed out.
• Last cell in a row. Every row holds exactly one sentinel. When a row has only one cell left that is not crossed out, that cell must be the sentinel.
• Neighbour squeeze. Sentinels never touch, even diagonally. When a row’s candidates are packed into a narrow band, any cell in the row above or below that would touch every one of those candidates can be crossed out.

Every one of these is explained properly in the Sentinels guide.