We must enforce a few invariants. For every pair of messages A, B:
* ownseqnum(A) ≤ ownseqnum(B) <=> ≡ seqnum(A) ≤ seqnum(B) <=> ≡ seqnum(parent(A)) ≤ seqnum(parent(B)
In practice, and because currently we implicitly allocate seqnums, for each incoming message m we only need to check: