.
                              
                                  f
                        l   ­    l    o        l
                    o          g   f            o
                  g               f                 g
                 f                 l                   f
                l                  o                   l
                o                 g                    o
                g              f       l               g
                 f             o      g               f
                  l              f     l               l
                    o              o               o
                      g            f               g
                        f           l             f
                           l        o         l
                             o     g      o
                                g  f   g
                                    l
                         ­           o
                                    g