The GFDL requires the ability to ""copy and distribute the Document in any medium, either commercially or noncommercially"" and therefore is incompatible with material that excludes commercial re-use. Material that restricts commercial re-use is incompatible with the license and cannot be incorporated into the work. However, incorporating such restricted material may be fair use under United States copyright law and does not need to be licensed to fall within the GFDL if such fair use is covered by all potential subsequent uses. One example of such liberal and commercial fair use is parody.