.
                                        C
                     ­            a    ac     a
                               c       tu        c
                              t          s           t
                             u       C  a          u
                            s        c     t          s
                            C       u     s          C
                            a        C    a         ­ a
                             c         c  t           c
                               t         u           t
                                  u      s       u
                                      s    :    s