.
                                     B
                            a    a   m    a
                          m     b   i        m
                         b          B            b
                       i             a               i
                      B            M                B
          ­          a           b        i             a
                   m          B         a            m
                    b            M      b         ­    b
                     i                  i                  i
                      B  ­             B               B
                        a              a             a
                          m          m         m
                            b          b        b
             ­                    i      i      i
                                        B
       ­                                 a
                              ­          m
                                        b
           ­                             i