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