public function isReserved(string $slug): bool
    {
        $slug = mb_strtolower($slug);
        $slug = str_replace(['-', '_', '.'], '', $slug);

        if (($is = &$this->cacheKey(__FUNCTION__, $slug)) !== null) {
            return $is; // Cached this already.
        }
        if (mb_strlen($slug) < 3) {
            return $is = true;
        } elseif (in_array($slug, $this::SLUG_RESERVED_STRINGS, true)) {
            return $is = true;
        }
        foreach ($this::SLUG_RESERVED_REGEX_FRAGS as $_regex_frag) {
            if (preg_match('/^'.$_regex_frag.'$/ui', $slug)) {
                return $is = true;
            }
        } // unset($_regex_frag); // Housekeeping.

        return $is = false;
    }