.
              
                                    L
          ­               a       a  m      a
                     m         b  L           m
                    b             a                 b
                   L              m                 L
                   a               b                   a
                  m            L   a                m
                  b             m  b                b
                   L            L   a                L
                    a            m b              a
                     m            L             m
                         b         a          b
                                    m
              ­                       b