1 2 3 4 5 6 7 8 9 +---------+---------+---------+ 1| . . . | . . . | . . . | 2| / X / | / / / | / X / | 3| . . . | . . . | . . . | +---------+---------+---------+ 4| . . . | . . . | . . . | 5| X X X | / / / | / X / | 6| . . . | . . . | . . . | +---------+---------+---------+ 7| . . . | . . . | . . . | 8| X X X | / / / | X X X | 9| . . . | . . . | . . . | +---------+---------+---------+