Really can't have two different and indistinguishable definitions of MUST. Suggest for the time being that the 1.4 definition be dropped. There is a way to achieve this goal but this isn't the way. There should be a declaration of a vocabulary which is then profiled and profiles stricter than the vocabulary enable interchange, something that isn't likely with part 1. In fact it would be nearly miraculous for two documents drafted under Part 1 to interoperate with stylesheets or other XML based applications. Use of XML is insufficient in and of itself to provide interchange.