.
                                       L
                               i      i a      i
                            a        r L        a
                           r          i a           r
                          L            r             L
                         i           L    i           i
                         a          a     r          a
                          r          L    i         ­ r
                             L        a r         L
                                  i      ~      i
             ­                            a
                                   ­      r