+---+     +---+     +---+
| b | <-- | a | --> | d |
+---+     +---+     +---+
  |         |         |
  |         |         |
  |         v         |
  |       +---+       |
  |       | c |       |
  |       +---+       |
  |         |         |
  |         |         |
  |         v         |
  |       +---+       |
  +-----> | u | <-----+
          +---+