fn parse_request(mut parts: std::str::SplitWhitespace<'_>) -> RequestParserResult<'_> {

        let songid1 = parts.next().ok_or(RequestParserError::UnexpectedEOF)?;

        let songid1 = songid1

            .parse()

            .map_err(|_| RequestParserError::SyntaxError(0, songid1.to_string()))?;

        let songid2 = parts.next().ok_or(RequestParserError::UnexpectedEOF)?;

        let songid2 = songid2

            .parse()

            .map_err(|_| RequestParserError::SyntaxError(0, songid2.to_string()))?;

        debug_assert!(parts.next().is_none());

        Ok((Request::SwapId(songid1, songid2), ""))

    }

    fn parse_response(

        parts: ResponseAttributes<'_>,

    ) -> Result<Self::Response, ResponseParserError> {

        debug_assert!(parts.is_empty());

        Ok(())

    }