Suppose that two individuals jointly own the copyright in a work of authorship. After one of the two sues a third party for infringement of the work, the defendant settles with that co-owner under an arrangement that includes a retroactive license, which purports to authorize any prior infringing use of the work by the defendant. Does this settlement thereafter absolve the alleged infringer of liability to the other copyright co- owner, on the theory that a joint owner of a copyright may unilaterally license use of the work? Or, should each co-owner be allowed to separately and independently enforce liability against the same defendant?