protected function fillWithEmptyCells($table_row,$tag_row)
  {
    $html = '';

    if( ! ($table_row instanceof TableLine) )
    {
      throw new \InvalidArgumentException;
    }

    $length_row = $table_row->getLength();
    $diff = $this->max_length_rows - $length_row;

    if( $diff > 0 )
    {
      // add empty cells
      foreach( range(1,$diff) as $key)
      {
        $html.="\t\t\t<{$tag_row}></{$tag_row}>\n";
      }
    }

    $html.="\t\t</tr>\n";

    return $html;
  }