public static function changeStatus($start, $end)
	{
		if ( empty($start) || ! is_string($start)) {
			throw new WorkflowException('$start must be a string');
		}

		if ( empty($end) || ! is_string($end)) {
			throw new WorkflowException('$end must be a string');
		}

		return 'from {'.$start.'} to {'.$end.'}';
	}