not also infringe another patent. The first patent in a given area might include a broad claim covering a general inventive concept if there is at that point no relevant prior art. Later, a specific implementation of that concept might be patentable if it is not disclosed in the earlier patent (or any intervening prior art), but nevertheless still falls within the scope of the earlier claim (covering the general concept). This means that the later inventor must obtain a license from the earlier inventor to be able to exploit their invention. At the same time, the earlier inventor might want to obtain a license from the later inventor, particularly if the later invention represents a significant improvement in the implementation of the original broad concept. In this case, the two enter into a cross license. Thomas Edison's thin carbon filament light bulb was a patentable improvement over the earlier patented Woodward and Evans thick carbon filament light bulb. Thomas Edison bought the Woodward patent for $5,000 US before he began his development work so that Woodward would not be able to later sue him for patent infringement after Edison became commercially successful. National laws United States Under United States patent law, inventorship is also regarded as a patentability criterion. It is a constitutional requirement. Congress's ability to grant patents is authorized only for the inventor. This was confirmed by case law: "Inventorship is indeed relevant to patentability under 35 U.S.C. § 102(f), and patents have in the past been held unenforceable for failure to correctly name inventors in cases where the named inventors acted in bad faith or with deceptive intent." Details on patentability in the U.S. can be found in the Manual of Patent Examining Procedure or MPEP. This is published by the United States Patent and Trademark Office (USPTO) and is the reference manual used by both patent examiners and patent agents/attorneys. Chapter 2100, in particular, gives a comprehensive overview of the standards for patentability, a discussion of the related case law, and guidance on how to overcome an examiner's rejection of a given set of claims. In the United States, the patent grant is presumptive, e.g. a patent shall issue unless the patent statutes preclude the grant. In other words, the burden is on the Patent Office to prove why a patent should not be granted. Once a patent issues, however, it is presumed valid and