                          Y                  X
                  ___..--' `._           _.-' `--..___
                 X            C         A             Y
             _.-' `._       _' \       / \        _.-' `._
            A        B     [Y]  []    []  [X]    B        C
           / \     _' \                        _' \     _' \
          []  [X] [X]  [Y]                    [X]  [Y] [Y]  []
 
