 Most countries place some limits on the patenting of inventions involving software, but there is no one legal definition of a software patent.For example, U.S. patent law excludes "abstract ideas", and this has been used to refuse some patents involving software.In Europe, "computer programs as such" are excluded from patentability, thus European Patent Office policy is consequently that a program for a computer is not patentable if it does not have the potential to cause a "technical effect" which is by now understood as a material effect (a "transformation of nature").