code: def _set_text(self, value):
          
          working_index = self.working_index
          working_lines = self._working_lines
  
          original_value = working_lines[working_index]
          working_lines[working_index] = value
  
          # Return True when this text has been changed.
          if len(value) != len(original_value):
              # For Python 2, it seems that when two strings have a different
              # length and one is a prefix of the other, Python still scans
              # character by character to see whether the strings are different.
              # (Some benchmarking showed significant differences for big
              # documents. >100,000 of lines.)
              return True
          elif value != original_value:
              return True
          return False
docs: set text at current working_index. Return whether it changed.