+---------+ +---------+
| v v |
+---+ +---+ +---+ +--------+ +---+
| G | <-- | | --> | | --> | C | --> | D |
+---+ | A | | B | +--------+ +---+
| | | | ^
| | --> | | ------+
+---+ +---+
|
|
v
+---+
| F |
+---+