+---+     +---+     +---+
| b | --- | a | --- | d |
+---+     +---+     +---+
  |         |         |
  |         |         |
  |         |         |
  |       +---+       |
  |       | c |       |
  |       +---+       |
  |         |         |
  |         |         |
  |         |         |
  |       +---+       |
  +-------| o |-------+
          +---+