.
                                   F
                            u    ul        u
                          l        lF           l
                        l          ul             l
                       F           l               F
                      u         F    u           u
                      l          l      l             l
                       l          F   u            l
                        F         l    l           F
                         u          F           u
                           l         u          l
                              l       l        l
                                      l