A key protector for a storage volume is created by generating an intermediate key and protecting, based at least in part on a publicprivate key pair, the intermediate key. An operator displaces the spark plug guard from the spark-plug covering position to the spark-plug opening position, thereby obtaining a wide working space to perform the maintenance and inspection of the spark plug efficiently. The plug connector includes at least first, second and third electrical contacts for electrically contacting the first, second and third pins, respectively, when the plug is inserted in the plug connector. The compound of the invention is found to be a cholinergic ligand at the nicotinic acetylcholine receptors.