():87
                               |
                +--------------+--------+
                |                       |
              ():41                   ():46
        +-------+-------+         +-----+--------+
        |               |         |              |
      ():20            e:21      f:22          ():24
   +----+----+                              +----+----+
   |         |                              |         |
  a:10      b:10                           c:12      d:12