docs(v4.0.0): step 6d -- the plan; and the wait offers on each port, not one

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/v4.0.0/MESH.md

@@ -492,34 +492,41 @@ ideas."
 
 ### 7c.2 The engine
 
-Two more addresses follow a node's ports (section 4.1, 7a):
+More addresses follow a node's ports (section 4.1, 7a):
 
 | Address | What |
 |---|---|
-| `base + V4_PORTS + 4` | Offer to: store the number of a port |
-| `base + V4_PORTS + 5` | Offer: store the word offered. Fetch: the wait |
+| `base + V4_PORTS + 4 + k` | Offer on port *k*: store the word offered there |
+| `base + 2 * V4_PORTS + 4` | The wait: a fetch here blocks |
 
-- A store to either is remembered and waits for nothing.
+*(Changed while the plan was written, 2026-10-07: as first approved there
+was one offer, on one port. A node may owe NACKs to several neighbours at
+once, and with one offer the node would sleep offering the first to a
+neighbour that is stuck while a second neighbour waited for its own. So
+a node may offer a word on each of its ports at once.)*
+
+- A store to an offer address is remembered and waits for nothing.
 - **A fetch of the wait blocks the node** until one of these, which the
   fabric looks for once a step, in this order:
   1. *A word for it.* A neighbour is blocked writing to it, on any port,
      or a device on one of its ports has a word to give. It gets that
      word, as a read of "any port" would, and "which port" says where
-     from. Its offer is withdrawn.
-  2. *Its offer taken.* The node on the port offered to is blocked reading
-     that port or any port; or is a device that takes what is written; or
-     is itself in the wait. The word is handed over. The fetch gives 0,
-     and "which port" gives `V4_PORTS`, which is no port: the offer was
-     taken.
+     from.
+  2. *An offer taken.* The node on a port it has an offer on is blocked
+     reading that port or any port; or is a device that takes what is
+     written; or is itself in the wait. The word is handed over. The
+     fetch gives 0, and "which port" gives `V4_PORTS + k`: the offer on
+     port *k* was taken. The lowest such port is the one.
   3. *Two nodes each offering to the other:* the one at the lower place in
      the fabric has its offer taken, and the other gets the word (case 2
      for the one, case 1 for the other).
+- Whichever it is, every offer the node had is withdrawn.
 - A node in the wait that is handed a word by case 2 of another node gets
   it by its own case 1.
 - A node in the wait executes nothing and spends no heat. It shows to its
   neighbours as waiting to read.
-- With nothing on the port offered to, a node that can take an error has
-  error 18 raised on it (section 4.1); a bare node waits. A neighbour that
+- With nothing on a port it has an offer on, a node that can take an error
+  has error 18 raised on it (section 4.1); a bare node waits. A neighbour that
   is asleep takes nothing: the offer stands.
 - `v4_fabric_interrupt_error`, a device's `pending`, and
   `v4_node_words_since_look` are removed.
@@ -533,9 +540,10 @@ Two more addresses follow a node's ports (section 4.1, 7a):
   instead, take that message in and offer again; when the offer is taken,
   write the rest, each word waiting for the neighbour, which has taken the
   first and reads to the end.
-- *Paying what is owed* (`(PAY)`): each NACK is offered the same way. A
-  node with nothing to do that owes one sleeps in the wait; it no longer
-  goes round, and it no longer writes one without looking.
+- *Paying what is owed* (`(PAY)`): every NACK owed is offered at once,
+  each on the port it goes by (one to a port). A node with nothing to do
+  that owes one sleeps in the wait; it no longer goes round, and it no
+  longer writes one without looking.
 - *Text from its console for a node that is waiting to begin a message*
   ends what the node was doing in error 21, "Interrupted", and is then
   done, as in `AWAIT` (7b.6). This is how Hera is typed to while she waits