protected function throwNewInvalidArgumentException ($errorMsg) {
		$selfClass = version_compare(PHP_VERSION, '5.5', '>') ? self::class : __CLASS__;
		$str = '['.$selfClass.'] ' . $errorMsg . ' ('
			. 'form id: `'.$this->id . '`, '
			. 'form type: `'.get_class($this->form).'`'
		.')';
		throw new \InvalidArgumentException($str);
	}