.
                                   O
                             r   r   g   r
                          g      a   s     g
                        a          m          a
               ­       s             O             s
                    m             r                m
                   O               g                  O
                  r             a       s                r
                 g             m        O               g
                 a              r         g                a
                  s               a      s                 s
                   m               m                   m
                     O               O                 O
                       r              r                r
                         g            g             g
                            a         a         a
                                 m  m   m
                                       O
                                       r
                                       g
                                       a
                                       s
                                      m