/** @brief Defines new reply helper format.
  * @param $format [in] new format to set (see AfsHelperFormat)
  * @return current instance.
  * @exception InvalidArgumentException when provided format is invalid.
  */
 public function set_helper_format($format)
 {
     AfsHelperFormat::check_value($format, 'Invalid helper format: ');
     $this->helper_format = $format;
     return $this;
 }