6.F.28: six paradoxes in a ring, and none of them seen

The fix for 6.F.22 asked whether a cycle had come back round to this order
before letting it take the two guesses. Necessary but not sufficient: it
looked only at the dependencies added since that order started.

6.F.28 is six convoy paradoxes arranged in a ring, each a tidy four-order
cycle in its own right. Every one of them satisfied that test, settled
itself locally against a caller's provisional answer, and wrote the result
down as final. The ring they were links in was never seen at all -- no
backup rule, no Szykman, and an answer that was locally consistent six times
over and globally wrong.

An order may now only settle a cycle when nothing anywhere in the dependency
list is still waiting further down the stack. If one of its own callers is
entangled, the answer is that caller's to settle, because their value is the
one everything here was computed against.

6.G.19 in the previous commit and this leaves one deviation of the hundred
and thirty-nine. 6.F.29 is 6.F.28 plus a single support that unwinds the
whole ring, which is what the document keeps the pair for. The half of the
ring furthest from that support comes out right and the half nearest it does
not: the resolver settles the first order it reaches while a second free
value elsewhere in the ring reads the same under both of its guesses, and
declares a determinacy it has not earned. Both halves are consistent on
their own. Together they are not.

README.md

@@ -107,7 +107,7 @@ published cases is parsed and run.
 
 | Section | | Passing |
 | --- | --- | --- |
-| 6.A - 6.G | movement | 136 / 139 |
+| 6.A - 6.G | movement | 138 / 139 |
 | 6.H | retreating | 18 / 18 |
 | 6.I | building | 7 / 7 |
 | 6.J | civil disorder | 13 / 13 |
@@ -121,13 +121,12 @@ Three cases are excluded rather than failed. 6.A.6, 6.B.10 and 6.B.11 state
 their setup in prose -- "Germany has a fleet in London" -- and the fixture is
 built from order lines, so it cannot carry a unit nobody ordered.
 
-Three are known deviations, written down so they are not rediscovered as
+One is a known deviation, written down so it is not rediscovered as a
-surprises:
+surprise:
 
 | Case | What it is |
 | --- | --- |
-| 6.F.28, 6.F.29 | Sixth order paradoxes: the same twenty-six orders with one extra support between them, which the document uses to show that the answer swings on it. The resolver gets five of the six sub-paradoxes right in both and the sixth wrong, and gives the same answer to both positions when the point is that they differ. These are positions nobody assembles by accident. |
+| 6.F.29 | The butterfly effect. Twenty-seven orders in a ring of six convoy paradoxes, plus one support that unwinds the whole thing -- and the document uses the pair 6.F.28/6.F.29 precisely to show how little it takes. 6.F.28 passes. In 6.F.29 the half of the ring furthest from the extra support comes out right and the half nearest it does not: the resolver settles the first order it reaches while a second free value elsewhere in the ring happens to read the same under both of its guesses, so it declares determinacy it has not earned. Both halves are individually consistent; together they are not. |
-| 6.G.19 | Whether an unnecessary fleet in a convoy chain signals intent. The document gives three different answers by edition, calls the question "a little bit pedantic", and advises that web adjudicators should not offer the order at all -- which is the line this one takes. |
 
 ### What the failing cases were worth
 
@@ -149,17 +148,34 @@ the case turns on the difference: the first holds a unit down, the second
 supports a move nobody can legally make. The extractor now keeps the
 distinction; it was losing it.
 
-**The head of a cycle (6.F.22).** Kruijswijk's resolver guesses that an order
+**The head of a cycle (6.F.22, 6.F.28).** Kruijswijk's resolver guesses that
-fails, sees what follows, and guesses the other way. Only the *outermost*
+an order fails, sees what follows, and guesses the other way. Only the
-order in a cycle may do that, because its answer is the one everything else
+*outermost* order in a cycle may do that, because its answer is the one
-was computed against. This engine let any order that reached the dependency
+everything else was computed against. This engine let any order that reached
-list first declare the cycle its own -- and an inner one takes both guesses
+the dependency list first declare the cycle its own -- and an inner one
-with its callers' provisional answers held fixed, gets the same result twice
+takes both guesses with its callers' provisional answers held fixed, gets
-for that reason, and records it as settled. The cycle then becomes
+the same result twice for that reason, and records it as settled. The cycle
-invisible: the backup rule never runs, and the position resolves to
+then becomes invisible: the backup rule never runs, and the position
-whichever of its two consistent readings the search walked into first. It
+resolves to whichever of its two consistent readings the search walked into
-had been doing that on every paradox and getting away with it, because on a
+first. It had been doing that on every paradox and getting away with it,
-first-order paradox both readings agree.
+because on a first-order paradox both readings agree.
+
+Fixing it took two passes. The first asked only whether the cycle had come
+back round to this order, which fixed 6.F.22. 6.F.28 needed the second: an
+order may not settle a cycle while *any* entry in the dependency list is
+still waiting further down the stack, not merely the entries added since it
+started. Six paradoxes in a ring, each a tidy four-order cycle in its own
+right, and each was settling itself locally against a caller's provisional
+answer. The ring they were links in was never seen at all.
+
+**A superfluous convoy order (6.G.19).** For a convoy order the fleet has to
+be necessary to some route -- there must be a route, ordered or not, that
+needs it. The path finder now considers only *proper* routes, which never
+jump to a province an earlier fleet in the chain already reached: Tunis -
+Ionian - Adriatic - Albania is not one, because the Ionian touches Albania
+and the Adriatic is doing nothing. It decides whether two units swap places
+or bounce off each other, and the document's own advice is that an
+interactive system should never offer the order.
 
 Sections H, I and J -- retreating and the winter -- have engine code but no
 harness yet; their cases are shaped differently and need one.

src/game/adjudicate.ts

@@ -368,12 +368,21 @@ function resolveAll(
     /*
      * Am I the outermost order of this cycle?
      *
-     * Only if the cycle came back round to me at all, and only if none of
+     * Two conditions. The cycle has to have come back round to me at all --
-     * its other members is still waiting further down the stack. Anything
+     * otherwise I am merely standing next to one. And nothing anywhere in
-     * else reports what it has and lets the one below ask.
+     * the dependency list may still be waiting further down the stack: if
+     * one of my own callers is entangled in this, the answer is theirs to
+     * settle, because their value is the one everything here was computed
+     * against.
+     *
+     * The second test looks at the whole list rather than the part added
+     * since I started, and the difference is the whole of 6.F.28. Six
+     * paradoxes in a ring, each a neat four-order cycle of its own; each one
+     * settled itself locally against a caller's provisional answer and
+     * recorded it as final, and the ring they were links in was never seen.
      */
     const cycle = dep.slice(mark)
-    if (!cycle.includes(p) || cycle.some((q) => below.has(q))) {
+    if (!cycle.includes(p) || dep.some((q) => below.has(q))) {
       stack.pop()
       dep.push(p)
       result.set(p, first)