"All that would be required is a rule that barred enforcement of a patent that was not reduced to practice within a specified time after the patent was granted" Actually, it would be better if the EXISTING rule that "a patent must describe the reduction to practice" were enforced! A patent isn't a valid patent if it doesn't teach a POSITA. If the applicant has never done it, then how is he to teach someone else to do it? To be valid, the applicant needs to have reduced the invention to practice BEFORE APPLYING for the patent. (I know I know - lots of patents have been granted before reduction to practice - but that is the big problem with the American patent system - most patents are wrongfully granted.)