+---------------------------------------------------+
|                        SM                         |
+---------------------------------------------------+
|                                                   |
|     +--------+   A   +--------+   B   +--------+  |
| *-->|   S0   |------>|   S1   |------>|   S2   |  |
|     +--------+       +--------+       +--------+  |
|         ^                                  |      |
|         |                 C                |      |
|         +----------------------------------+      |
|                                                   |
+---------------------------------------------------+
