code: def find_probable_year_index(self, tokens):
          
          for index, token in enumerate(self):
              potential_year_tokens = _ymd.find_potential_year_tokens(
                  token, tokens)
              if len(potential_year_tokens) == 1 and len(potential_year_tokens[0]) > 2:
                  return index
docs: attempt to deduce if a pre 100 year was lost
           due to padded zeros being taken off