1                                                                                       
                                                                                                                                                       
                                                                                                                                                       
                                                                                                                                                       
                                                                                                                                                      3
                                                                                                                                                       
                                     exists?                                                                no                                         
                                                                                                                                                       
                                                                         yes                                                                           
                                                                                                                                                       
                                                                                                                                                       
                                                                                                                                                      4
                                                               2