+---+
+------ | u | -+
| +---+ |
| | |
| | |
v v |
+---+----+----+ |
| a | b | c | |
+---+----+----+ |
^ |
+---------+
+---+
+------ | u | -+
| +---+ |
| | |
| | |
v v |
+---+----+----+ |
| a | b | c | |
+---+----+----+ |
^ |
+---------+