b11
                                                                                                                                                                                                                                                  
                                                                                                                                                                                                                                                  
                                                                                                                                                 b1                                                                                               
                    root                                                                                                                                                                                                                          
                                                                                                                                                                                                                                                  
                                                                                                                                                                                                                                               b12
                                                                                                                                                                                                                                                  
                                                                                                                                                 b2