The aim of the game is to remove all tiles. you can remove a matching
pair of tiles by clicking on the first to mark it red and then on the
second. Both tiles may not be blocked when you want to remove it. A
tile is free if the stack left or right of the tile is lower than the
own. The height of a stack of tiles is indicated by the gray border
left and below each tile. The thicker the border the higher the stack.