I don't completely understand what you mean, here, @mhvk: you mean instead of `frame_attr_names`, there'd be a `frame_attributes` object that acts like a `dict`, but instead of holding the actual values, it points to values in the `FK5` object's `__dict__`? Or that we should use the values in `__dict__` *itself*? I'm fine with the first (although I'd appreciate a pointer to where that is in `Quantity` subclasses so that there's no need to re-implement), but I don't think the second will work, because there are attributes in `__dict__` that would *not* necessarily be `frame_attributes`