.

                                       E
                               a     a s      a
                            s         y E        s
                           y          a s          y
                          E            y            ­ E
                          a          E  a            a
        ­                 s           s    y           s
                          y         E    a          y
                           E         s  y          E
                              a        E         ­ a
                                s       a       s
                                    y   s    y
                                         y