+---++---+
| U || C |
+---++---+
^ ^
| |
| |
+---++---+ |
| A || B | |
+---++---+ |
| | |
+----+----+
+---++---+
| U || C |
+---++---+
^ ^
| |
| |
+---++---+ |
| A || B | |
+---++---+ |
| | |
+----+----+