code: def make_stream_tls_features(self, stream, features):
          
          if self.stream and stream is not self.stream:
              raise ValueError("Single StreamTLSHandler instance can handle"
                                                              " only one stream")
          self.stream = stream
          if self.settings["starttls"] and not stream.tls_established:
              tls = ElementTree.SubElement(features, STARTTLS_TAG)
              if self.settings["tls_require"]:
                  ElementTree.SubElement(tls, REQUIRED_TAG)
          return features
docs: Update the <features/> element with StartTLS feature.
  
          [receving entity only]
  
          :Parameters:
              - `features`: the <features/> element of the stream.
          :Types:
              - `features`: :etree:`ElementTree.Element`
  
          :returns: update <features/> element.
          :returntype: :etree:`ElementTree.Element`