Attain the highest possible score by removing as many balls as possible.
Any horizontal, vertical or diagonal line of three balls disappears immediatly,
and the balls above fall down.
New balls fall in from the top as well. If the new ones form a line it disappears too.
If nothing is falling anymore you can swap two balls that are horizontally or vertically next to each other,
but you may swap two balls only if you form a new line.
The game is finished when you can not form a new line by swapping two balls.