.
                                   T
                          a      a c      a
                       c         o T         c
                     o           a c            o
                   T              o               T
                  a             ­  T                 a
                  c                a         ­         c
                o                  c                  ­ o
                T                T a                  T
                a              c      o                a
                 c             T      a                c
                   o             c   o               o
                     T              T              T
                       a             a          a
                           c         c        c
                                      o
              ­                        T
                                      a­
                                      c
                        ­              o