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proof(ordinal): unbudgeted sound-carrier recursive surface wf-<ᵇʳᶠ² (#212)
## What The recursive-surface route to well-foundedness **without the ℕ budget**, realised over the doubled-ladder sound carrier `_<ᵇ²_` instead of native `_<ᵇ_`. New module `RecursiveSurfaceSound.agda`: ```agda _<ᵇʳᶠ²_ : BT → BT → Set -- _<ᵇ²_ core + ψα/+2 congruences rank2-mono-<ᵇʳᶠ² : s <ᵇʳᶠ² t → rank2 s <′ rank2 t wf-<ᵇʳᶠ² : WellFounded _<ᵇʳᶠ²_ -- NO budget ``` ## Context — this is roadmap item #1, in the only form that is achievable The roadmap's #1 ordinal item is "eliminate the ℕ budget from `wf-<ᵇʳᶠᵇ`". The **global** version (over native `_<ᵇ_`) is documented-**walled**: `RankBrouwer.agda`'s preamble + `buchholz-rank-obstruction.adoc` record all five standard routes (rank, direct-mutual, lex, tower, inverse-image) as blocked, because `<ᵇʳᶠ-core` inherits native `_<ᵇ_`'s ordinal *unsoundness* (the `<ᵇ-+Ω` counterexample `bplus bzero (bOmega (fin 1)) <ᵇ bOmega (fin 0)` admits no rank). This session's `rank2` does **not** escape that — the same counterexample defeats it. The documented **"recommended next move 1"** is the WF-restricted relation. This module realises it: building the recursive surface over the **sound carrier `_<ᵇ²_`** makes all three cases rank2-embeddable, so WF follows by the standard rank transport with no budget. The budget was an artefact of native unsoundness, not of the same-binder recursion itself. ## How - `<ᵇʳᶠ²-core` → `rank2-mono-<ᵇ²` (this session's umbrella). - `<ᵇʳᶠ²-ψα` → `⊕-mono-<-right` on `rank2 α <′ rank2 β` (since `rank2 (bpsi ν α) = ω-rank-pow (double ν) ⊕ rank2 α`). - `<ᵇʳᶠ²-+2` → `⊕-mono-<-right` on `rank2 y <′ rank2 z` (since `rank2 (bplus x y) = rank2 x ⊕ rank2 y`). The latter two are exactly the two congruence discharges the `RankBrouwer` preamble already identified as `⊕-mono-<-right`; the doubled ladder supplies the previously-missing core case. Then `wf-<ᵇʳᶠ² = Subrelation.wellFounded rank2-mono-<ᵇʳᶠ² (On.wellFounded rank2 wf-<′)`. ## Honest scope `_<ᵇʳᶠ²_` is to `_<ᵇʳᶠ_` what `_<ᵇ²_` is to native `_<ᵇ_` — the ordinally-sound restriction. It does **not** claim the global unbudgeted `wf-<ᵇʳᶠ` over native `_<ᵇ_` (that remains walled). Its contribution: the recursive-surface route is well-founded *without* the ℕ budget once the core is the sound carrier. ## Verification - `agda -i proofs/agda proofs/agda/All.agda` exits 0 under `--safe --without-K`. - Headlines pinned in the Buchholz `Smoke.agda` (own block). - Zero postulates / escape pragmas; structural recursion (no `TERMINATING`). https://claude.ai/code/session_017t53M7W7ubmXpwymveLcCE --- _Generated by [Claude Code](https://claude.ai/code/session_017t53M7W7ubmXpwymveLcCE)_ Co-authored-by: Claude <noreply@anthropic.com>
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‎proofs/agda/All.agda‎

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@@ -166,6 +166,7 @@ open import Ordinal.Buchholz.RankMonoUnion
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open import Ordinal.Buchholz.RankMonoUnionWF
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open import Ordinal.Buchholz.RankMonoUnionWfCNF
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open import Ordinal.Buchholz.RecursiveSurfaceOrder
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open import Ordinal.Buchholz.RecursiveSurfaceSound
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open import Ordinal.Buchholz.RecursiveSurfaceBudget
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open import Ordinal.Buchholz.SurfaceOrder
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open import Ordinal.Buchholz.VeblenObligations

‎proofs/agda/Ordinal/Buchholz/RecursiveSurfaceSound.agda‎

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{-# OPTIONS --safe --without-K #-}
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-- SPDX-License-Identifier: MPL-2.0
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-- SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
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-- Unbudgeted recursive-surface descent over the SOUND carrier
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-- (2026-06-14).
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--
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-- ## What this lands
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--
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-- `_<ᵇʳᶠ²_` — the recursive same-binder closure over the doubled-ladder
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-- sound carrier `_<ᵇ²_` (`RankDoubledLadderUmbrella`) instead of native
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-- `_<ᵇ_`, with its UNBUDGETED well-foundedness:
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--
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-- * `rank2-mono-<ᵇʳᶠ² : s <ᵇʳᶠ² t → rank2 s <′ rank2 t`
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-- * `wf-<ᵇʳᶠ² : WellFounded _<ᵇʳᶠ²_`
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--
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-- ## Why the budget is no longer needed
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--
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-- The existing `RecursiveSurfaceBudget._<ᵇʳᶠᵇ_` carries an explicit ℕ
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-- depth budget precisely because its `_<ᵇʳᶠ_` is built over NATIVE
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-- `_<ᵇ_`, whose `<ᵇʳᶠ-core` case is ordinally UNSOUND: no rank embeds
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-- it (the `<ᵇ-+Ω` counterexample `bplus bzero (bOmega (fin 1)) <ᵇ
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-- bOmega (fin 0)` forces `rank-mono` to invert). See
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-- `RankBrouwer.agda`'s preamble and `buchholz-rank-obstruction.adoc`:
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-- the rank / direct-mutual / lex / tower / inverse-image routes to the
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-- GLOBAL unbudgeted `wf-<ᵇʳᶠ` over native `_<ᵇ_` are all walled, and
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-- `rank2` does not escape it (the same counterexample defeats `rank2`).
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--
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-- The documented "recommended next move 1" is the WF-RESTRICTED
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-- relation. This module realises it: building the recursive surface
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-- over the SOUND carrier `_<ᵇ²_` makes all three cases rank2-embeddable,
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-- so well-foundedness follows by the standard rank transport WITHOUT a
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-- budget:
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--
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-- * `<ᵇʳᶠ²-core` — `rank2-mono-<ᵇ²` (this session's umbrella);
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-- * `<ᵇʳᶠ²-ψα` — `⊕-mono-<-right` on `rank2 α <′ rank2 β`
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-- (`rank2 (bpsi ν α) = ω-rank-pow (double ν) ⊕ rank2 α`);
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-- * `<ᵇʳᶠ²-+2` — `⊕-mono-<-right` on `rank2 y <′ rank2 z`
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-- (`rank2 (bplus x y) = rank2 x ⊕ rank2 y`).
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--
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-- The latter two are exactly the two congruence cases the
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-- `RankBrouwer` preamble already identified as discharged by
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-- `⊕-mono-<-right`; the doubled ladder supplies the missing core case.
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--
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-- ## Honest scope
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--
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-- `_<ᵇʳᶠ²_` is the sound-carrier recursive surface — it is to
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-- `_<ᵇʳᶠ_` what `_<ᵇ²_` is to native `_<ᵇ_`: the ordinally-sound
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-- restriction. It does NOT claim the GLOBAL unbudgeted `wf-<ᵇʳᶠ` over
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-- native `_<ᵇ_` (that remains walled per the obstruction note). Its
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-- contribution is that the recursive-surface route is well-founded
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-- WITHOUT the ℕ budget once the core is the sound carrier — the budget
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-- was an artefact of native unsoundness, not of the same-binder
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-- recursion itself.
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--
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-- ## Headlines (pin in `Ordinal/Buchholz/Smoke.agda`)
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--
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-- * `_<ᵇʳᶠ²_`
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-- * `rank2-mono-<ᵇʳᶠ²`
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-- * `wf-<ᵇʳᶠ²`
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module Ordinal.Buchholz.RecursiveSurfaceSound where
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open import Induction.WellFounded using (WellFounded; module Subrelation)
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open import Relation.Binary.Construct.On as On using (wellFounded)
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open import Ordinal.Brouwer using (Ord)
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open import Ordinal.Brouwer.Phase13 using (_<′_; ⊕-mono-<-right; wf-<′)
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open import Ordinal.Brouwer.Arithmetic using (_⊕_)
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open import Ordinal.Buchholz.Syntax using (BT; bpsi; bplus)
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open import Ordinal.Buchholz.RankPow using (ω-rank-pow)
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open import Ordinal.Buchholz.RankDoubledLadder using (rank2; double)
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open import Ordinal.Buchholz.RankDoubledLadderUmbrella using
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( _<ᵇ²_
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; rank2-mono-<ᵇ²
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)
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----------------------------------------------------------------------
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-- The sound-carrier recursive surface `_<ᵇʳᶠ²_`
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----------------------------------------------------------------------
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-- Native `_<ᵇ_` replaced by the sound carrier `_<ᵇ²_` in the core
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-- rule; the two same-binder congruence rules are unchanged.
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infix 4 _<ᵇʳᶠ²_
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data _<ᵇʳᶠ²_ : BT → BT → Set where
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<ᵇʳᶠ²-core : ∀ {x y} → x <ᵇ² y → x <ᵇʳᶠ² y
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<ᵇʳᶠ²-ψα : ∀ {ν α β} → α <ᵇʳᶠ² β → bpsi ν α <ᵇʳᶠ² bpsi ν β
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<ᵇʳᶠ²-+2 : ∀ {x y z} → y <ᵇʳᶠ² z → bplus x y <ᵇʳᶠ² bplus x z
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----------------------------------------------------------------------
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-- rank2 strict-monotonicity — no budget
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----------------------------------------------------------------------
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-- All three cases land directly: the core via the doubled-ladder
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-- umbrella, the two congruences via right-strict-monotonicity of `⊕`
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-- (the ψ-block leading power / the bplus head are the fixed left
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-- summand). Structural recursion on the `_<ᵇʳᶠ²_` derivation.
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rank2-mono-<ᵇʳᶠ² : ∀ {s t} → s <ᵇʳᶠ² t → rank2 s <′ rank2 t
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rank2-mono-<ᵇʳᶠ² (<ᵇʳᶠ²-core p) = rank2-mono-<ᵇ² p
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rank2-mono-<ᵇʳᶠ² (<ᵇʳᶠ²-ψα {ν} {α} {β} p) =
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⊕-mono-<-right {ω-rank-pow (double ν)} {rank2 α} {rank2 β}
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(rank2-mono-<ᵇʳᶠ² p)
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rank2-mono-<ᵇʳᶠ² (<ᵇʳᶠ²-+2 {x} {y} {z} p) =
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⊕-mono-<-right {rank2 x} {rank2 y} {rank2 z}
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(rank2-mono-<ᵇʳᶠ² p)
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----------------------------------------------------------------------
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-- Unbudgeted well-foundedness via the rank2 embedding
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----------------------------------------------------------------------
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-- Step 1 — InverseImage transport of `wf-<′` along `rank2`.
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wf-rank2-pullback : WellFounded (λ x y → rank2 x <′ rank2 y)
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wf-rank2-pullback = On.wellFounded rank2 wf-<′
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-- Step 2 — Subrelation transport: `rank2-mono-<ᵇʳᶠ²` witnesses that
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-- `_<ᵇʳᶠ²_` is a sub-relation of the pullback. NO ℕ budget.
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wf-<ᵇʳᶠ² : WellFounded _<ᵇʳᶠ²_
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wf-<ᵇʳᶠ² = Subrelation.wellFounded rank2-mono-<ᵇʳᶠ² wf-rank2-pullback

‎proofs/agda/Ordinal/Buchholz/Smoke.agda‎

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@@ -521,6 +521,15 @@ open import Ordinal.Buchholz.RankDoubledLadderUmbrella using
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; wf-<ᵇ²
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)
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-- Unbudgeted sound-carrier recursive surface (own block per CLAUDE.md
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-- Working rules): the recursive same-binder closure over `_<ᵇ²_` and
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-- its budget-free well-foundedness.
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open import Ordinal.Buchholz.RecursiveSurfaceSound using
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( _<ᵇʳᶠ²_
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; rank2-mono-<ᵇʳᶠ²
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; wf-<ᵇʳᶠ²
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)
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-- Slice 3 prerequisites (own block per CLAUDE.md Working rules):
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-- the left-spine NonBzero predicate, the strict-jump bridge from
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-- `μ <Ω ν` to `ω-rank-pow-succ μ ≤′ ω-rank-pow ν`, and the head-Ω

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