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