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| . . . | . . . | . . . |
+---------+---------+---------+