

   +------+------
   |      |
 +-+-+  +-+-+
 | a |  | b | 
 +---+  +---+
   |      |
   +------+------
   |      |
 +---+  +---+
 | X |  | Y | 
 +-+-+  +-+-+
   |      |
   +------+------
