This is just like regular Stack4,
but the object is to make your opponent be the first to connect four or
more pieces in a row.
There is one other rule: you can not place a piece on top of the piece
your opponent just placed on the board in the previous move.
If the board fills up with pieces and nobody has put four in a row,
then "x" wins. This is because "x" has a disadvantage as the first to move.
