function _flushGroup($new_tag) {
  if ($this->_group !== '') {
    if ($this->_tag == 'mark') {
      $this->_line .= '<span class="diffchange">' . check_plain($this->_group) . '</span>';
    }
    else {
      $this->_line .= check_plain($this->_group);
    }
  }
  $this->_group = '';
  $this->_tag = $new_tag;
}