The third category of works, those of particular concern to us here, are those that entered the collection via a sale, donation, or barter arrangement that prohibited commercial use, reserved for the photographers the right in perpetuity to borrow the works for exhibition and publication purposes, did not specify transfer of ownership, and otherwise contain explicitly or implicitly encumbering language. Here too the court should determine this by examination of the relevant contracts and letters of agreement covering those works, whose sale to buyers not bound by the initial understandings between the Polaroid Corporation and the photographers would in fact constitute breach of contract. […]