public function normalizeAmps(string $qs_url_uri): string
    {
        if (!$qs_url_uri) {
            return $qs_url_uri; // Possible `0`.
        }
        if (($regex_amps = &$this->cacheKey(__FUNCTION__.'_regex_amps')) === null) {
            $regex_amps = implode('|', array_keys($this::HTML_AMPERSAND_ENTITIES));
        }
        return preg_replace('/(?:'.$regex_amps.')/uS', '&', $qs_url_uri);
    }