.
                                  s
                          e­    e   x     e
                        x      s   e        x
                      s          xs            s
                     e            e               e
                    x             x                x
                   s            s     e              s
                  e           x       s              e
                 x            e       x              x
                  s             s     e               s
                   e               x                 e
                     x             s                x
                       s            e            s
                          e         x         e
                             x      s     x
                                     e
                                     x