
       +-------------------+
       |                   v
     +---+     +---+     +---+
     | 1 | --> | 2 | --> | 3 |
     +---+     +---+     +---+
+---+----+----+
| 5 |  6 |  7 |
+---+----+----+
+---+----+
| 3 |  4 |
+---+----+
