@Dominic

If you have two out-of-order messages by the same author, o1 and o2, you can't prove that o2 really signs o1 - but what is that even useful for? you only know or care that these messages exist because of your friend, so I suggest that we've already captured the important thing.

Doesn't that argumentation apply to ssb as a whole? Why do we even bother with backlinks, we can just use sequence numbers then.

Speaking of ooo: I can't see the message %eDQzaPn... that Dominic has replied to, and patchwork doesn't have ooo yet. Could somebody paste that message and/or link to the author?