async def cancel(self):
        """Coroutine to cancel this request/stream.

        Client will send RST_STREAM frame to the server, so it will be
        explicitly informed that there is nothing to expect from the client
        regarding this request/stream.
        """
        if self._cancel_done:
            raise ProtocolError('Stream was already cancelled')

        with self._wrapper:
            await self._stream.reset()  # TODO: specify error code
            self._cancel_done = True