Rules: The challenge is to paint each cell black or white, subject to the following rules:
- Each numbered cell is a wall cell, the number in it is the number of cells in that wall.
- Each wall must contain exactly one numbered cell.
- There must be only one stream, which is not allowed to contain "pools", i.e. 2x2 areas of black cells.
- All centre squares, starting with the 2x2 square and working outwards produce gradually larger Nurikabes of their own.