-- What the mesh last sent a machine, as a digest.
--
-- Not the declaration itself: the mesh can compute that again at any moment, and keeping a copy
-- would be a second account of what a machine should be, able to disagree with the first. What
-- cannot be recomputed is *what was actually sent*, and that is the whole of the difference
-- between "this machine is out of date" and "this machine has never been told".
--
-- Without it, "behind" could only mean "failed or refused" — so a machine that applied cleanly and
-- whose declaration has since changed was not behind, and novox/hq ADR 0010's question, *did my
-- change go out?*, was answerable only for machines that broke.

alter table node add column sent text;
-- When, so a machine sent something long ago and silent since is distinguishable from one sent
-- something a moment ago that has not had time to answer.
alter table node add column sent_at timestamptz;