Ok. What about: The attribute is `version_in_use` and the argument of the save method is `use_version` or may be only `version` or `specific_version`. (I don't like the argument use_version so much because inside the save method you call self.use_version and it sounds like a command but it isn't.) From outside, you are right: `motion.save(use_version=...)` sounds like what we want to get here.