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