b                                                            
                                                                                                       a b c                                                        
                                                                                                                                                                    
                                                                                                                                                                    
                                                                                                                                                                    
                                                                                                                                                                    
                    a                                                                                                                                              c
                                                                                                       a b c                                                        
                                                                                                                                                                    
                                                                                                                                                                    
                                                                                                                                                                    
                                                                                                                                                                    
                                                                                                                                                                   d
                                                                                                       a b c