Untitled

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

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

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