3 Colour every square either black or white, in such a way that:
4 <ul><li>no three consecutive squares, horizontally or vertically, are
6 <li>each row and column contains the same number of black and white
9 Left-click in an empty square to turn it black, or right-click to turn
10 it white. Click again in an already-filled square to cycle it between
11 black and white and empty; middle-click to reset any square to empty.