.

                                       F
                             r      r   e      r
                           e        a   k        e
                         a            F              a
                       k              r                k
                       F              e                 F
                      r                k                 r
                      e            F       r            e
                      a            e       a            a
                       k             k   F             k
                        F              r                F
                          r            e   ­           r
                            e          a            e
                                 k              k