+---------------------+
             |                     |
             |                     |
  +----------+----------+          |
  |          |          v          v
+----+     +----+     +----+     +----+
| E1 | --> | E2 | --> | S1 | --> | S2 |
+----+     +----+     +----+     +----+
  |                                ^
  +--------------------------------+