diff --git a/README.md b/README.md
index 7a32906..f808711 100644
--- a/README.md
+++ b/README.md
@@ -16,7 +16,7 @@ the same discipline as the four `*-ed25519-verified` subject corpora.
 | L5 | inclusion soundness = EXPLICIT extractor `extractIncl` (Theorem 2) **done, non-vacuous** |
 | L6a | descent extractor `extractMTH` (Theorem 3 step 3 = Lemma 2, WHOLE-TREE instance) **done, non-vacuous** |
 | — | Lemma 2, PATH instance (receipt-uniqueness of `Root`): deleted with the vacuous `root_binding` in S3.5 and **not yet restored**; optional, not needed for Theorem 3 |
-| L6b | ConsRec binding + Theorem 3 assembly (extractCons) | pending (S5) |
+| L6b | ConsRec binding + Theorem 3 assembly (extractCons) | in progress (S5): stage-1 arithmetic/list infrastructure done (kbelow_prefix_eq, take surgery); binding + assembly next |
 | L6 | pin-store state machine safety (Proposition 1) | pending |
 
 ## Discipline (identical to the subject corpora)
diff --git a/verification/Proofs/AxiomCheck.lean b/verification/Proofs/AxiomCheck.lean
index 8c12f5e..202c3fa 100644
--- a/verification/Proofs/AxiomCheck.lean
+++ b/verification/Proofs/AxiomCheck.lean
@@ -3,6 +3,7 @@ import Proofs.Basic
 import Proofs.Completeness
 import Proofs.Extract
 import Proofs.Descent
+import Proofs.Consistency
 #print axioms LTLAcc.domsep
 #print axioms LTLAcc.kbelow_pos
 #print axioms LTLAcc.kbelow_lt
@@ -21,3 +22,6 @@ import Proofs.Descent
 #print axioms LTLAcc.extractMTH
 #print axioms LTLAcc.extractMTH_correct
 #print axioms LTLAcc.extractMTH_nonvacuous
+#print axioms LTLAcc.kbelow_prefix_eq
+#print axioms LTLAcc.take_take_le
+#print axioms LTLAcc.take_drop_prefix
diff --git a/verification/Proofs/AxiomCheck.olean b/verification/Proofs/AxiomCheck.olean
index 4e32568..1952ad9 100644
Binary files a/verification/Proofs/AxiomCheck.olean and b/verification/Proofs/AxiomCheck.olean differ
diff --git a/verification/Proofs/Consistency.lean b/verification/Proofs/Consistency.lean
new file mode 100644
index 0000000..c91af52
--- /dev/null
+++ b/verification/Proofs/Consistency.lean
@@ -0,0 +1,67 @@
+/- S5 (stage 1) — infrastructure for consistency soundness (paper Theorem 3,
+   steps 1-2). The binding argument needs one non-obvious arithmetic fact and
+   two list-surgery facts, isolated and proven here before the main proof.
+
+   Arithmetic: when the pinned prefix of size n₀ spills past the left subtree
+   (k < n₀ ≤ n, k = kbelow n), the honest tree of `D₁.take n₀` splits at the
+   SAME point k. That is `kbelow n₀ = k`, and it holds because k is a power of
+   two with k < n₀ ≤ 2k, which pins kbelow uniquely. -/
+import Proofs.Descent
+
+namespace LTLAcc
+
+/-- Two powers of two both lying in `(m/2, m]` (i.e. `2^a < m ≤ 2^(a+1)`)
+    have the same exponent. -/
+theorem pow2_exp_unique {a j m : Nat}
+    (ha : 2^a < m) (ha2 : m ≤ 2^(a+1)) (hj : 2^j < m) (hj2 : m ≤ 2^(j+1)) :
+    a = j := by
+  rcases Nat.lt_trichotomy a j with h | h | h
+  · -- a < j ⇒ a+1 ≤ j ⇒ 2^(a+1) ≤ 2^j < m, contradicting m ≤ 2^(a+1)
+    have : a + 1 ≤ j := h
+    have hle : 2^(a+1) ≤ 2^j := Nat.pow_le_pow_right (by omega) this
+    omega
+  · exact h
+  · have : j + 1 ≤ a := h
+    have hle : 2^(j+1) ≤ 2^a := Nat.pow_le_pow_right (by omega) this
+    omega
+
+/-- `kbelow` is pinned by its defining inequalities: a power of two `p`
+    with `p < m ≤ 2p` IS `kbelow m`. -/
+theorem kbelow_eq_of_pow2_between {p m : Nat} (j : Nat) (hp : p = 2^j)
+    (h1 : p < m) (h2 : m ≤ 2 * p) : kbelow m = p := by
+  have hm2 : 2 ≤ m := by
+    have : 1 ≤ p := by rw [hp]; exact Nat.one_le_two_pow
+    omega
+  obtain ⟨a, ha⟩ := kbelow_pow2 m
+  have hlt := kbelow_lt m hm2
+  have hle := le_two_kbelow m hm2
+  -- kbelow m = 2^a with 2^a < m ≤ 2^(a+1); p = 2^j with 2^j < m ≤ 2^(j+1)
+  rw [ha] at hlt hle
+  have hj2 : m ≤ 2^(j+1) := by rw [Nat.pow_succ]; omega
+  have ha2 : m ≤ 2^(a+1) := by rw [Nat.pow_succ]; omega
+  have hja : 2^j < m := by omega
+  have : a = j := pow2_exp_unique hlt ha2 hja hj2
+  rw [ha, this, ← hp]
+
+/-- The specialization used in the binding: with `k = kbelow n`, `2 ≤ n`,
+    and `k < n₀ ≤ n`, the prefix tree splits at the same `k`. -/
+theorem kbelow_prefix_eq {n n₀ : Nat} (hn : 2 ≤ n)
+    (hk : k = kbelow n) (hlo : k < n₀) (hhi : n₀ ≤ n) :
+    kbelow n₀ = k := by
+  obtain ⟨j, hj⟩ := kbelow_pow2 n
+  rw [← hk] at hj
+  have hle := le_two_kbelow n hn
+  rw [← hk] at hle
+  exact kbelow_eq_of_pow2_between j hj hlo (by omega)
+
+/-! ### list surgery -/
+
+theorem take_take_le (l : List Bytes) (k n₀ : Nat) (h : k ≤ n₀) :
+    (l.take n₀).take k = l.take k := by
+  rw [List.take_take]; congr 1; omega
+
+theorem take_drop_prefix (l : List Bytes) (k n₀ : Nat) :
+    (l.take n₀).drop k = (l.drop k).take (n₀ - k) := by
+  rw [List.drop_take]
+
+end LTLAcc
diff --git a/verification/Proofs/Consistency.olean b/verification/Proofs/Consistency.olean
new file mode 100644
index 0000000..50cc899
Binary files /dev/null and b/verification/Proofs/Consistency.olean differ
diff --git a/verification/check.sh b/verification/check.sh
index 2616687..d09cb4a 100755
--- a/verification/check.sh
+++ b/verification/check.sh
@@ -17,7 +17,7 @@ export LEAN_MEM_MB="${LEAN_MEM_MB:-4096}"
 CORES="${LEAN_MAX_CORES:-0-3}"
 
 GEN_MODULES=( LTLAcc/HashExternal )
-PROOFS=( Basic Completeness Extract Descent )
+PROOFS=( Basic Completeness Extract Descent Consistency )
 
 # Certificates and their exact expected cones (observed at first green
 # compile, 2026-07-10; any drift in EITHER direction is a failure).
@@ -40,6 +40,9 @@ declare -A CONES=(
   [LTLAcc.extractMTH]="propext, LTLAcc.sha256, Quot.sound"
   [LTLAcc.extractMTH_correct]="propext, Classical.choice, LTLAcc.sha256, Quot.sound"
   [LTLAcc.extractMTH_nonvacuous]="propext, LTLAcc.sha256, Quot.sound"
+  [LTLAcc.kbelow_prefix_eq]="propext, Quot.sound"
+  [LTLAcc.take_take_le]="propext, Quot.sound"
+  [LTLAcc.take_drop_prefix]="propext, Classical.choice, Quot.sound"
 )
 
 free -m | awk '/Mem:/{if($7<2048){print "FATAL: <2GB RAM available — refusing to compile"; exit 1}}'