Copy writing music is really easy and cheap to do. Simply record your song, It doesn't have to be great quality, just good enough to represent how it would sound. Put it on a disc, then write down the chords and the melody if you can. Put the disc and the paper into an envelope, then post it to yourself.
When you receive the envelope in the post, it will be sealed with a postmark that is dated, thus protecting your work by giving you proof in a court of when you wrote your song.
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