3 Tile the rectangle with dominoes (1×2 rectangles) so that
4 every possible domino appears exactly once (that is, every possible
5 pair of numbers, including doubles).
7 Click between two adjacent numbers to place or remove a domino.
8 Right-click to place a line between numbers if you think a domino
9 definitely cannot go there. Dominoes light up red if two identical
10 ones appear on the grid.