public static function addWord($word, $freq, $tag = '', $options = array())
    {
        if (isset(self::$original_freq[$word])) {
            self::$total -= self::$original_freq[$word];
        }
        self::$original_freq[$word] = $freq;
        self::$total += $freq;
        $l = mb_strlen($word, 'UTF-8');
        $word_c = array();
        for ($i=0; $i<$l; $i++) {
            $c = mb_substr($word, $i, 1, 'UTF-8');
            array_push($word_c, $c);
        }
        $word_c_key = implode('.', $word_c);
        self::$trie->set($word_c_key, array("end"=>""));
        self::__calcFreq();
        self::$dag_cache = array();
        return self::$trie;
    }