.
                                        B
                                 l     l o     l
                              o       s s        o
                             s        om         s
                            s           B            s
                            o       l       o        o
                            m      s      s        m
                             B       o   m        B
                               l        B  l         l
                                o        o        o
                                   s      s      s
                                          O
                                          M