I agree that a multilateral treaty would be particularly problematic, and I think, in the first instance, Canada should try its own approach. We've been doing a pretty good job. We may be able to improve it. There are certain unilateral measures that we can take. Again, I've listed them on page 2.
With respect to this multilateral treaty, you're correct in pointing out the difficulties. In fact, the Convention on Mutual Administrative Assistance--the one I'm trying to get Canada to sign.... There are only, currently, 16 signatories. I think Germany and Canada have yet to ratify the agreement. It's only enforced for 14 people, and it's been a number of years since that convention was first created.
International taxes is particularly an interesting area to study from a public international law perspective, because in all other areas we've seen over the last half century the rise of multilateral agreements, world trade organization agreements, but now we only have bilateral agreements in Canada and elsewhere to preserve tax sovereignty.