Untitled

+---+---+---+
|     F   X | X |
+   +---+---+---+
| X | X   X | F |
+---+---+   +   +
| F | F | X   X |
+   +   +---+   +

+---+---+---+
| X | B | X   F |
+   +---+---+---+
| F | X   X | B |
+---+   +   +---+
| B | B   F   X |
+---+---+---+   +

+---+---+---+
| X | X   X | B |
+   +---+---+---+
| B | X   X | X |
+---+   +---+   +
| X | X | B | X |
+   +   +   +   +