public function add_keyword($key, $word) {
        if (!is_array($this->language_data['KEYWORDS'][$key])) {
            $this->language_data['KEYWORDS'][$key] = array();
        }
        if (!in_array($word, $this->language_data['KEYWORDS'][$key])) {
            $this->language_data['KEYWORDS'][$key][] = $word;

            //NEW in 1.0.8 don't recompile the whole optimized regexp, simply append it
            if ($this->parse_cache_built) {
                $subkey = count($this->language_data['CACHED_KEYWORD_LISTS'][$key]) - 1;
                $this->language_data['CACHED_KEYWORD_LISTS'][$key][$subkey] .= '|' . preg_quote($word, '/');
            }
        }
    }