                               |
                               y                |
                             <-->               w
                   ___...---'    \             <0>
                   x              []       _.-'   `._
                  <+>                =>   x          y
                 /   `._                 / \       _' \
                []      w               []  [w]   [w]  []
                      _' \
                     [x]  [y]
 
