docs: def tokens(self):
          """
          `generator` : the tokens in this segment
          """
          for subsegment_or_token in self:
              if isinstance(subsegment_or_token, Segment):
                  subsegment = subsegment_or_token
                  for token in subsegment.tokens():
                      yield token
              else:
                  token = subsegment_or_token
                  yield token