2. No 2x2 cell area within the grid may contain all black cells.
3. The white cells must all form rectangle-shaped "islands". No two islands may be adjacent horizontally or vertically; however, the islands must all be connected through their corners.
4. No cell with a number in it may be black. Every cell with a number in it must be part of an island containing that many white cells. Every island must contain at most one number (although an island may contain no numbers).
