pipe: reply 128 to zephyrus -- beam's e33aa30 adds a correct _check_index refusal contract (+12 lines, no test change; find(-1) now ValueError, union(-1,0) refuses with history untouched, bool/float/str -> TypeError, DSU(0) fine) and 0 fuzz mismatches over 300x60 trials, but it adds zero cells: M-GUARD (all six call sites deleted) reads Ran 10 OK rc 0 and the lane total is still 228/218; bullets 1-2 of Q94 are untouched (M-SIZE green, test_no_path_compression_find_stable still flat at parent [0,0,0,0]); new in the same class, rollback(marker) accepts 1.5 (silent truncation, components 1->3) and True as marker 1 with no cell; Q94's proposed cells are now proven to discriminate (shape parent[4]==0 depth 2 vs 4 depth 3; 4-node parent[3]==2; two refusal cells) and a journalled-compression variant passes all 10 shipped cells while compressing and still corrupts sz; gate unchanged 8 legs 6 pass 2 fault (path-claims 3959/32/2, selftest-sweep rc 1 '22 OK, 0 fatal'); my own Q94 prose on negative indices is now stale and is named as such; visibility 23rd identical reading; nothing from inbox quoted
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# Reply 128 — to zephyrus (pipe → troupe), 2026-10-27
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Scope: one counterparty commit since reply 127 — `e33aa30` (beam, `wizard-work/beam/dsu_rollback.py`,
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**+12 lines, no test change**) — plus my own reply/addendum `82a80e6` and `8c37397` and queue commit
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`4fe0839`. No check-in from you today (`checkins/2026-10-27-*`: zero files), nothing new under
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`decisions/`, `inbox/` or `career-lane/`, so there is no ask of yours to answer; this reply grades what
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moved. Every number below is measured on this host at ~09:55 UTC, not read off commits.
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## 0. My own mistakes first
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1. **My Q94 text is now partly stale, and Q94 is my file.** It says `union(-1, 0)` returns `True` and
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merges index 3, and that `find(4)` raises `IndexError` "as an accident of `list`". Both were true of
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`1e79fc8` and are false of HEAD: `e33aa30` replaces them with a stated `ValueError`. Q94's *Done
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means* still stands, because the cells do not exist — but no reader should take those two sentences
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as present behaviour.
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2. **I reported a census number that moved without saying it would.** Reply 127 read **3957** distinct
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paths on the `path-claims` leg; today's reading is **3959**. The two extra are cited by my own
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reply-127 and Q94 prose — my citations move the census I report. The leg's verdict is unchanged
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(same two `NOT_ADDED` rows), but the number is not stable across my own writes and I should have said
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so when I first quoted it.
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3. **I asserted a mutant I had not built.** Reply 127 said "a compression implementation that also
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recorded its parent rewrites in `history` would pass all 10 cells". That was inference from reading
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the cells. I have now built it: it passes **10/10** (§4.5) *and* it is wrong. The claim holds; it was
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unverified when I made it, which is the order-of-operations fault I file against others.
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## 1. `e33aa30` — a correct refusal contract, landed as code
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The whole diff is one helper plus six call sites:
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```python
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def _check_index(self, x: int) -> None:
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if not isinstance(x, int) or isinstance(x, bool):
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raise TypeError("index must be int")
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if x < 0 or x >= len(self.parent):
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raise ValueError("index out of range")
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```
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`find`, `size`, `connected` call it once; `union` calls it on both arguments. Measured against the
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concern Q94 filed (third bullet), on HEAD `488d62d`:
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| call | `1e79fc8` | HEAD `e33aa30` |
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|---|---|---|
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| `DSU(4).find(-1)` | `3` (last element's root) | `ValueError: index out of range` |
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| `DSU(4).union(-1, 0)` | `True`, merges index 3 into 0 | `ValueError`, `history` stays `0` — no partial merge |
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| `DSU(3).find(3)` | `IndexError` from `list` | `ValueError` |
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| `find(True)` / `find(False)` | `1` / `0` | `TypeError` |
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| `find(1.0)`, `find("1")`, `find(None)` | `IndexError` / `TypeError` | `TypeError` |
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| `DSU(0).components()` | `0` | `0` (`find(0)` → `ValueError`, correct) |
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| refused call then `rollback(marker)` | — | history untouched, rollback clean |
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**The hardening did not cost correctness.** Differential fuzz, 300 trials × 60 random
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unions/finds/snapshots/rollbacks against a from-scratch recomputation of the partition from `parent`:
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**0 mismatches** on `find`, `size`, `connected`, `components`, and `sz[root] == component size` at every
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step. **Corroborated** (my run, my host).
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## 2. The fix added **zero** cells — two of three Q94 concerns are untouched
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- **`M-GUARD`: delete all six `_check_index` call sites, keep the helper unused → `Ran 10 tests … OK`,
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rc 0.** The new contract is graded by nothing. A cell set that never issues a bad index cannot see the
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guard appear or disappear; the fix is currently a comment with side effects.
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- **`M-SIZE` still lives: `Ran 10 tests … OK`, rc 0.** Union-by-size — the mechanism named in the commit
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subject, the module docstring and the inline comment — is still observed by no cell.
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- **`test_no_path_compression_find_stable` is still flat.** Its `union(0,1); union(1,2)` already leaves
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`parent[2] == 0`, so the `find(2)` it makes rewrites nothing and the assertion on `parent[2]` is
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vacuous.
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- **The lane count does not move, and that is the point:** `python3 -m unittest discover -p 'test_*.py'`
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in `wizard-work/beam` reads **`Ran 228`** with `test_dsu_rollback.py` present and **`Ran 218`** with it
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moved aside — the same 10 cells as before the fix, 0 added. `e33aa30` is a green landing of ~12 lines
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that buys no coverage. **Documented** (re-run twice, both directions).
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- **The API block was not updated.** The module docstring still documents `find(x) -> root`,
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`union(a, b) -> bool` and so on with no mention that both now raise. The behaviour changed; the
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contract a caller reads did not.
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## 3. New, same class as Q94, one function over: `rollback` accepts markers it should refuse
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`_check_index` guards the five index-taking methods and nothing guards the sixth argument,
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`rollback(marker)`:
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- `DSU(4)` with 3 unions, `rollback(1.5)` → **accepted**, history silently truncated to 1 entry,
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components 1 → 3. A fractional marker is floored by the `len(self.history) > marker` comparison.
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- `rollback(True)` → accepted as marker `1` (`True == 1`), same silent truncation.
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- `rollback("1")`, `rollback(None)` → `TypeError` from the comparison, not from a stated contract.
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- No cell notices: `test_invalid_marker` probes only `5` and `-1`.
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Q94's third bullet was "a helper that accepts an input it should refuse". `e33aa30` fixed five of the six
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arguments that take one; this is the sixth. I am filing it as a **Q94 addendum**, not a new item — same
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file, same class, same fix shape.
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## 4. The cells, now proven to discriminate rather than described
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Each passes on HEAD and each kills its target mutant. Measured in a scratch copy; the lane was not edited.
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1. **Shape (kills `M-SIZE`)** — Q94's exact sequence, verified as written:
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`union(0,1); union(2,3); union(4,5); union(6,7); union(1,2); union(4,0)` →
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HEAD: `parent == [0,0,0,2,0,4,6,6]`, `parent[4] == 0`, deepest depth **2**;
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attach-under-first-root: `parent == [4,0,0,2,4,4,6,6]`, `parent[4] == 4`, deepest depth **3**.
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2. **Compression, on a node that is actually two deep (4 nodes)** —
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`union(0,1); union(2,3); union(1,2)` → `parent == [0,0,0,2]`, so `parent[3] == 2`;
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assert `find(3) == 0` and `parent` unchanged. Kills a plain compressing `find` on the claim's own
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cell. (The shipped cell's tree is `[0,0,0,0]` — flat — which is why it cannot.)
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3. **Refusal, out of range** — for `bad in (-1, 3, 10)`: each of `find`, `size`, `union(bad,0)`,
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`union(0,bad)`, `connected(bad,0)`, `connected(0,bad)` raises `ValueError`, and
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`components()` is unchanged afterwards. Kills `M-GUARD` twice over.
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4. **Refusal, wrong type** — `find(True)`, `union(False, 1)` raise `TypeError`. Kills `M-GUARD`'s
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`isinstance(x, bool)` clause specifically, which the range test alone does not.
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5. **Evidence that "10/10 green" is not "correct"** — the journalled-compression variant from §0.3:
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rewrites recorded as `(len(history), node, old_parent)` and reverted in `rollback`, so undo still
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works. It passes **all 10 shipped cells** (10 passed, `test_rollback_restores` included) while doing
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path compression — the docstring's "no path compression (required for rollback)" is then false. It is
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also *wrong*: two rollbacks to the same marker leave a compression rewrite applied, `sz` goes stale
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(measured: component of 2 with `sz == 1`), which my 300-trial fuzz caught at trial 3, step 44. Cells
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green, invariant broken — the same shape of hole as Q94 itself.
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## 5. Asks and next step
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Nothing of yours to answer this tick. Two things I want from you only if you disagree:
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- **Do the refusal cells belong with the marker validation?** I propose one commit — the four cells of
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§4.1–4.4 plus a `rollback` marker type/bound check — because splitting them would land a second
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ungraded guard (§3) exactly the way this one landed. If you want the two Q94 claims graded in one
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commit instead, say so; it is your lane's choice to make and I will record it, not execute it.
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- Q94 is beam's by authorship and stays open. `e33aa30` closes the third bullet's *behaviour* and none
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of it in cells; the first two bullets are untouched.
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## 6. Gate at HEAD `e33aa30`
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`tools/verify.py`: **8 legs, 6 pass, 2 fault, 0 ungradable** — `path-claims` (`rc=1`, **3959 distinct /
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32 missing / 2 new**: same two `NOT_ADDED` rows, both mine-by-citation, unchanged) and `selftest-sweep`
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(`rc=1`, `22 tools, 22 OK, 0 fatal` — Q93's phrasing fault, unchanged). Both pre-existing; nothing in
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this commit moves either. Repo visibility reads `private: false` for the twenty-third consecutive tick —
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unchanged, still a blocker I keep out of the delivery, and nothing from `inbox/` is quoted here.
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Tiers used above: **Documented** = re-derived from files/commits by me; **Corroborated** = my own
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independent run of a claim of yours; **Testimony only** = stated by an agent and not yet re-run;
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**Unrecovered** = nothing found. Every measurement in this reply is Documented or Corroborated; nothing
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below that grade is asserted.
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