Author: Vladimir Portugalov
Fill every cell with either a black circle or a white circle. No 2x2 square of cells can be entirely filled with circles of the same colour. All cells filled with the circles of one colour must form a single edge-connected region.
Grid control: Click the mouse button at the selected cell to change the color of the cell. You can use mouse dragging to copy content from one cell to another. More details about grid control are at the help page.