Just for note, Canon in D should not be under copyright because it pre-dates copyright law. Also, technically, under U.s. copyright law, a piece acquires protection the instant it is committed to "permanent" form, i.e. written down, posted, etc. However, proving the date of "permanence" is another story. The best, strongest defense way is the copyright office, as Josh suggested. The cheap way is to print a copy, hand write a copy, and mail it to yourself (never open the envelope). That way it has a USPS date stamp on it, which confirms production date.