I think it has to happen on both @certs, otherwise there will be serious confusion.
Agreed.
Syd, you seem to be arguing that data.certainty itself ought to enable both options -- are you?
No, not at all. I *like* data.probability just the way it is. And, while I'm not too fond of it, see no reason to change data.certainty. What I do think is that the alternation of the two of them is very sensible for many of these uses. It allows a project to choose one or the other. And it's quite an easy customization to make to say "our @cert attrs will always be data.probability". So, without the benefit of actually looking at them, my instinct is that many cases where we have one or the other, we should have the alternation of both. But I'd want to go through on a case-by-case basis to check that it makes sense. (Luckily, there are not too many cases.)