.
                                        P
                     ­          u      up        u
                             P        pet          p
                            p          P ­             p
                            e         u   p           e
                            t          p    e           t
                            p         t     P           P
                            u         u    p          u
                              p         p  e          p
                                p          t     ­      p
                                  e        P        e
         ­                            t      P     t