.
                                         b
                                  o    oo     o
                                o      bo       o
                              b         o          b
                              o       b   o        o
                              o       o    b        o
                              b       o    o        b
                               o       b   o        o
                                o       o b         o
                                  b       o        b
                                     o     o      o
                                        o  b   o
                                            o