Sentinels #123

• Last cell in a region. Every coloured region holds exactly one sentinel. A region down to a single open cell is solved.
• Region locked to a line. If every open cell of a region sits in one row, that region’s sentinel is somewhere in that row - so no other cell in the row can hold one. Cross the rest of the row 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.
• 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.

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