________                         ________
            |        |____                   |        |____    
      ______|             |            ______|             |   
     |                    |           |      |             |   
 ____|                    |  =>   ____|      |             | 
|                         |      |           | subproblem  |   
|                         |      | subproblem|             |   
|_________________________|      |___________|_____________|