Concurrent and consecutive sentencing: subsequent imprisonment begins after prior sentence unless court orders concurrency; life sentences run concurrently. When a person already undergoing imprisonment is sentenced again, the later imprisonment ordinarily begins at the expiration of the prior sentence unless the court directs concurrent operation. If the earlier imprisonment was imposed by an order under section 141 in default of furnishing security and the subsequent sentence is for an offence committed prior to that order, the subsequent sentence commences immediately. A subsequent sentence imposed on a person already serving life imprisonment runs concurrently with the existing life sentence.