public function headword($headword)
    {
        if ($headword instanceof Headword) {
            return $this->andWhere(['headword_guid' => $headword->guid]);
        }
        if (is_string($headword)) {
            $headword = Headword::find()->where(['word' => $headword])->one();
            if (!$headword) {
                return $this;
            }
            return $this->andWhere(['headword_guid' => $headword->guid]);
        }
    }