private function getRuleClass($rule) {

		if(false === is_string($rule)) {
			return trigger_error(sprintf('Argument 1 passed to %s() must be of the type string, "%s" given', __METHOD__, gettype($rule)), E_USER_ERROR);
		}

		if(true === isset($this -> custom[$rule])) {
			return $this -> custom[$rule];
		}

		return __NAMESPACE__ . self :: NAMESPACE_RULES . ucfirst(strtolower($rule));
	}