a                                                           b
a                                                            b
a                                                             b
a                                                              b
a                                                               b
a                                                                b
a                                                                 b
a                                                                  b
a                                                                   b
a                                                                    b
a                                                                     b
a                                                                      b
a                                                                       b
a                                                                        b
a                                                                         b
a                                                                          b
a                                                                           b
a                                                                            b
a                                                                             b
a                                                                              b
                                                                                
a
                                                                                
ab