.

                                     C
                           a      a   k        a
                        k         e   C           k
                      e              a                e
       ­              C               k                 C
                    a                e                   a
                   k                 C                    k
                   e               a     k                e
                   C              e      C                C
                   a               a     k                 a
                    k                 e                    k
                     e                 C                  e
                       C               a                C
                          a            k      ­       a
                             k         e         k
                                 e     C     e
                                         a
                                         k
                                         e