The failure guarded against is not subtle and is very hard to recover from: a
rule set that closes the hub's own port takes the private network down, and the
mesh's way of fixing anything is to send a declaration over it.
So the assertion that matters is not the rule file — it is that a declaration
still reaches the other machine afterwards, and that the other machine still
reaches the hub. A rule file that looks right and a mesh that has stopped are
exactly what this is for.