As I said, I was stating the view of the canonists, not endorsing it! To make matters even more ridiculous, what happens in the case of a Protestant who becomes Eastern Orthodox (or Nestorian, for that matter) and later wishes to become a Catholic?
I've argued this one with canon lawyers repeatedly, and the answer usually boils down to "well, that's the law even though we know we can't enforce it". Seems to me that a law which cannot be enforced does not bind.

Incognitus