3 Black out some of the squares, in such a way that:
4 <ul><li>no number appears twice in any row or column
5 <li>no two black squares are adjacent
6 <li>the white squares form a single connected group (connections
7 along diagonals do not count).</ul>
9 Click in a square to black it out, and again to uncover it.
10 Right-click in a square to mark it with a circle, indicating that
11 you're sure it should <em>not</em> be blacked out.