+---+
                     |   |
             +---+---+   +---+---+
             |   |           |   |
         +---+   +---+   +---+   +---+
         |           |   |           |
         +---+   +---+---+---+   +---+
         |   |   |           |   |
     +---+   +---+           +---+
     |           |
     +---+   +---+           +---+
         |   |   |           |   |
         +---+   +---+---+---+   +---+
         |           |   |           |
         +---+   +---+   +---+   +---+
             |   |           |   |
             +---+---+   +---+---+
                     |   |
                     +---+