.
                              P
                     a      a r     a
                   r        a d       r
                 a          o x         a
                d            P            d
               o             a              o
              x              r                x
­              P              a                 P
             a            d      o             a
             r           x        P             r
              a           a       r            a
               d           a    d           d
                 o             o            o
                        X      X      X
          ­                       X
                                 X