+---+
| A |
+---+
  |       +----+
  +-----> | A1 |
  |       +----+
  |       |    |
  +-----> | A2 |
  |       +----+
  |       |    |
  +-----> | A3 |
  |       +----+
  |       |    |
  +-----> | A4 |
          +----+