|
                              g               |
                             <b>              f
                         _.-'   \            <b>
                         f       d       _.-'   `_
                        <r>        =>    q         g
                    _.-'   \            <r>       <r>
                    q       c          /   \     /   \
                   <r>                a     b   c     d
                  /   \
                 a     b