.
                                    P
                         ­ a       a w        a
                         w       n P          w
                        n         a w           n
                       P           n               P
                       a          P   a            a
                       w        w   n           w
                        n        P     a          n
                          P       w  n          P
                           a         P           a
                             w      a        w
                               n     w     n
                                       n