+---+
| A |
+---+
  |       +----+
  +-----> | A1 |
  |       +----+
  |       +----+
  +-----> | A2 |
  |       +----+
  |         |        +-----+
  |         +------> | A23 |
  |         |        +-----+
  |         |        +-----+
  |         +------> | A22 |
  |         |        +-----+
  |         |        +-----+
  |         +------> | A21 |
  |                  +-----+
  |       +----+
  +-----> | A3 |
          +----+