protected function _store($domain, $header, $sentence)
    {
        if (!isset($this->_storage[$domain])) {
            $this->_storage[$domain] = [];
        }
        if (!isset($this->_storage[$domain][$sentence])) {
            $this->_storage[$domain][$sentence] = $header;
        } else {
            $this->_storage[$domain][$sentence] .= $header;
        }
    }