                          |                     |
                          s                     r
                        <++>                   <->
                      _'    `_             _.-'   \
                      a        r    =>     s       c
                     h-1      <0>         <+>      h
                             /   \      _'   \
                            b     c     a     b
                            h     h    h-1    h
 
