Tim and Mel are long distance lovers. Tim has just purchased an engagement ring for Mel and wants to mail it to her. Unfortunately the only way to ensure the ring will be received is to place a lock on the package. Tim has locks and Mel has locks but neither have keys for each others locks. How can they ensure the ring isn't stolen?
Tim places a lock on the package and sends it to Mel. Mel places one of her locks on the package and send it back to Tim. Tim removes his lock and sends the package back to Mel.
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