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