Round-tripping means that `unparse(parse(string)) == string`. See https://en.wikipedia.org/wiki/Round-trip_format_conversion. Most parsers aren't round tripping (e.g., most Python parsers will ignore syntactically insignificant whitespace). I don't think this one needs to be. It just matters here because we can't test that `unparse_url(parse_url(url)) == url` in these cases.