no                                 data?                                                                 
                                                                                                                                                                         
                                                                                                                               yes                                       
                                                                                                                                                                         
                                                                                                                                                                         
                                                                                                                                                                         
                                                                   read data                                                                                             
                                                                                                                                                                         
                                                                                                                                                                         
                                                                                                                                                                         
                                                                                                                                                                         
                                                                                                                                                                         
                                                                                                                                                                         
                                                                  a                                                                                         b            
                                                                                                                                                                         
                                                                            yes                                                                                       yes