Construct a sequence diagram for the following use case "Returning DVDs". DVDs can be rented and returned by mail. Here, we only model the use case of returning DVDs. After a customer finishes watching a set of DVDs, he/she mails them back via post office. The DVDs spend some time in transit. The post office delivers the DVDs to an agent of the rental company. The agent scans each DVD to get its information. The agent also retrieves the rental information associated with each DVD from company's system (database), so that the corresponding rental fee can be calculated. The rental information is then updated with the return date and the fee. The agent then creates an invoice to the customer. The DVDs are either sent to post office immediately to mail to customers if they are ordered, or restocked in the inventory (in such case the system is updated with the availability of these DVDs)