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