code: def start_response(self, stream_id, headers, *, end_stream=False):
          
          yield from self._resumed.wait()
          self._conn.send_headers(stream_id, headers, end_stream=end_stream)
          self._flush()
docs: Start a response by sending given headers on the given stream.
  
          This may block until the underlying transport becomes writable.
  
          :param stream_id: Which stream to send response on.
          :param headers: A list of key-value tuples as headers.
          :param end_stream: To send a response without body, set `end_stream` to
                             `True` (default `False`).