public function setTableExtraClasses($classes = array())
  {
    if( ! empty($classes) )
    {
      // validate classes
      foreach($classes as $class)
      {
        if(str_word_count($class) > 1)
        {
          throw new \InvalidArgumentException;
        }
      }
      $this->table_extra_classes = $classes;
    }
  }