.
                                    C
                         ­  o     o a      o
                        a        c h         a
                       c           C            c
                      h            o             h
                     C             a              C
                    o              c            ­    o
                   a               h                 a
                   c             C  o                c
                   h            a     c              h
                   C           h      C             C
                    o           o      a            o
                      a           c    h           a
                          c          C          c
             ­                h       o     h
                                      a
                       ­               c
                                      h