.

                                      P
                        ­    e      e    t       e
                          t         P   e          t
                        P             t               P
                       e              p                e
                      t        ­        e                  t
                     P                t                   P
                     e             P     e               e
                      t            t       P              t
                       P            e    t              P
                         e             P               e
                           t            e             t
                             P         t           P
                               e       P        e
                                    t   e    t
                                         t