    +-------------------+------+---------------------+
    |                   |      |                     |
    |                   |  S2  |                     |
    |                   |      |                     |
    |                   |      |                     |
    |                   +------+                     |
    |                   |      |                     |
    |       S1          |      |         S3          |
    |                   |      |                     |
    |                   +------+                     |
    |                   |      |                     |
    |                   |  S4  |                     |
    |                   |      |                     |
    |                   |      |                     |
    +-------------------+------+---------------------+
