.
                                    
                                        C
                      ­         u      u n       u
                            n          t C           n
                           t             u               t
                          C             n               C
                          u           C u              u
                          n          n     t            n
                           t           C   u            t
                             C         n  t           C
                               u         C          u
                                  n      u       n
                                     t    n    t
                                           t