diff --git a/src/Data/List/Relation/Binary/Permutation/Algorithmic.agda b/src/Data/List/Relation/Binary/Permutation/Algorithmic.agda
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+-------------------------------------------------------------------------------
+-- The Agda standard library
+--
+-- A alternative definition for the permutation relation using setoid equality
+-- Based on Choudhury and Fiore, "Free Commutative Monoids in HoTT" (MFPS, 2022)
+-- Constructor `_⋎_` below is rule (3), directly after the proof of Theorem 6.3,
+-- and appears as rule `commrel` of their earlier presentation at (HoTT, 2019),
+-- "The finite-multiset construction in HoTT".
+-------------------------------------------------------------------------------
+
+{-# OPTIONS --cubical-compatible --safe #-}
+
+open import Relation.Binary.Bundles using (Setoid)
+
+module Data.List.Relation.Binary.Permutation.Algorithmic
+  {s ℓ} (S : Setoid s ℓ) where
+
+open import Data.List.Base using (List; []; _∷_; length)
+open import Data.List.Properties using (++-identityʳ)
+open import Data.Nat.Base using (ℕ; suc)
+open import Data.Nat.Properties using (suc-injective)
+open import Level using (_⊔_)
+open import Relation.Binary.PropositionalEquality as ≡ using (_≡_)
+
+open import Data.List.Relation.Binary.Equality.Setoid S as ≋
+  using (_≋_; []; _∷_; ≋-refl)
+
+open Setoid S
+  renaming (Carrier to A; refl to ≈-refl; sym to ≈-sym; trans to ≈-trans)
+
+private
+  variable
+    a b c d : A
+    as bs cs ds : List A
+    n : ℕ
+
+
+-------------------------------------------------------------------------------
+-- Definition
+
+infix  4  _∼_
+infix  5 _⋎_ _⋎[_]_
+
+data _∼_ : List A → List A → Set (s ⊔ ℓ) where
+  []   : [] ∼ []
+  _∷_  : a ≈ b → as ∼ bs → a ∷ as ∼ b ∷ bs
+  _⋎_  : as ∼ b ∷ cs → a ∷ cs ∼ bs → a ∷ as ∼ b ∷ bs
+
+-- smart constructor for prefix congruence
+
+_≡∷_  : ∀ c → as ∼ bs → c ∷ as ∼ c ∷ bs
+_≡∷_ c = ≈-refl ∷_
+
+-- pattern synonym to allow naming the 'middle' term
+pattern _⋎[_]_ {as} {b} {a} {bs} as∼b∷cs cs a∷cs∼bs
+  = _⋎_ {as} {b} {cs = cs} {a} {bs} as∼b∷cs a∷cs∼bs
+
+-------------------------------------------------------------------------------
+-- Properties
+
+∼-reflexive : as ≋ bs → as ∼ bs
+∼-reflexive []            = []
+∼-reflexive (a≈b ∷ as∼bs) = a≈b ∷ ∼-reflexive as∼bs
+
+∼-refl : ∀ as → as ∼ as
+∼-refl _ = ∼-reflexive ≋-refl
+
+∼-sym : as ∼ bs → bs ∼ as
+∼-sym []                  = []
+∼-sym (a≈b     ∷ as∼bs)   = ≈-sym a≈b ∷ ∼-sym as∼bs
+∼-sym (as∼b∷cs ⋎ a∷cs∼bs) = ∼-sym a∷cs∼bs ⋎ ∼-sym as∼b∷cs
+
+≋∘∼⇒∼ : as ≋ bs → bs ∼ cs → as ∼ cs
+≋∘∼⇒∼ []            []                  = []
+≋∘∼⇒∼ (a≈b ∷ as≋bs) (b≈c ∷ bs∼cs)       = ≈-trans a≈b b≈c ∷ ≋∘∼⇒∼ as≋bs bs∼cs
+≋∘∼⇒∼ (a≈b ∷ as≋bs) (bs∼c∷ds ⋎ b∷ds∼cs) =
+  ≋∘∼⇒∼ as≋bs bs∼c∷ds ⋎ ≋∘∼⇒∼ (a≈b ∷ ≋-refl) b∷ds∼cs
+
+∼∘≋⇒∼ : as ∼ bs → bs ≋ cs → as ∼ cs
+∼∘≋⇒∼ []                  []            = []
+∼∘≋⇒∼ (a≈b ∷ as∼bs)       (b≈c ∷ bs≋cs) = ≈-trans a≈b b≈c ∷ ∼∘≋⇒∼ as∼bs bs≋cs
+∼∘≋⇒∼ (as∼b∷cs ⋎ a∷cs∼bs) (b≈d ∷ bs≋ds) =
+  ∼∘≋⇒∼ as∼b∷cs (b≈d ∷ ≋-refl) ⋎ ∼∘≋⇒∼ a∷cs∼bs bs≋ds
+
+∼-length : as ∼ bs → length as ≡ length bs
+∼-length []                  = ≡.refl
+∼-length (a≈b ∷ as∼bs)       = ≡.cong suc (∼-length as∼bs)
+∼-length (as∼b∷cs ⋎ a∷cs∼bs) = ≡.cong suc (≡.trans (∼-length as∼b∷cs) (∼-length a∷cs∼bs))
+
+∼-trans  : as ∼ bs → bs ∼ cs → as ∼ cs
+∼-trans = lemma ≡.refl
+  where
+  lemma : n ≡ length bs → as ∼ bs → bs ∼ cs → as ∼ cs
+
+-- easy base case for bs = [], eq: 0 ≡ 0
+  lemma _ [] [] = []
+
+-- fiddly step case for bs = b ∷ bs, where eq : suc n ≡ suc (length bs)
+-- hence suc-injective eq : n ≡ length bs
+
+  lemma {n = suc n} eq (a≈b ∷ as∼bs) (b≈c ∷ bs∼cs)
+    = ≈-trans a≈b b≈c ∷ lemma (suc-injective eq) as∼bs bs∼cs
+
+  lemma {n = suc n} eq (a≈b ∷ as∼bs) (bs∼c∷ys ⋎ b∷ys∼cs)
+    = ≋∘∼⇒∼ (a≈b ∷ ≋-refl) (lemma (suc-injective eq) as∼bs bs∼c∷ys ⋎ b∷ys∼cs)
+
+  lemma {n = suc n} eq (as∼b∷xs ⋎ a∷xs∼bs) (a≈b ∷ bs∼cs)
+    = ∼∘≋⇒∼ (as∼b∷xs ⋎ lemma (suc-injective eq) a∷xs∼bs bs∼cs) (a≈b ∷ ≋-refl)
+
+  lemma {n = suc n} {bs = b ∷ bs} {as = a ∷ as} {cs = c ∷ cs} eq
+    (as∼b∷xs ⋎[ xs ] a∷xs∼bs) (bs∼c∷ys ⋎[ ys ] b∷ys∼cs) = a∷as∼c∷cs
+    where
+    n≡∣bs∣ : n ≡ length bs
+    n≡∣bs∣ = suc-injective eq
+
+    n≡∣b∷xs∣ : n ≡ length (b ∷ xs)
+    n≡∣b∷xs∣ = ≡.trans n≡∣bs∣ (≡.sym (∼-length a∷xs∼bs))
+
+    n≡∣b∷ys∣ : n ≡ length (b ∷ ys)
+    n≡∣b∷ys∣ = ≡.trans n≡∣bs∣ (∼-length bs∼c∷ys)
+
+    a∷as∼c∷cs : a ∷ as ∼ c ∷ cs
+    a∷as∼c∷cs with lemma n≡∣bs∣ a∷xs∼bs bs∼c∷ys
+    ... | a≈c ∷ xs∼ys = a≈c ∷ as∼cs
+      where
+      as∼cs : as ∼ cs
+      as∼cs = lemma n≡∣b∷xs∣ as∼b∷xs
+               (lemma n≡∣b∷ys∣ (b ≡∷ xs∼ys) b∷ys∼cs)
+    ... | xs∼c∷zs ⋎[ zs ] a∷zs∼ys
+      = lemma n≡∣b∷xs∣ as∼b∷xs b∷xs∼c∷b∷zs
+        ⋎[ b ∷ zs ]
+        lemma n≡∣b∷ys∣ a∷b∷zs∼b∷ys b∷ys∼cs
+      where
+      b∷zs∼b∷zs : b ∷ zs ∼ b ∷ zs
+      b∷zs∼b∷zs = ∼-refl (b ∷ zs)
+      b∷xs∼c∷b∷zs : b ∷ xs ∼ c ∷ (b ∷ zs)
+      b∷xs∼c∷b∷zs = xs∼c∷zs ⋎[ zs ] b∷zs∼b∷zs
+      a∷b∷zs∼b∷ys : a ∷ (b ∷ zs) ∼ b ∷ ys
+      a∷b∷zs∼b∷ys = b∷zs∼b∷zs ⋎[ zs ] a∷zs∼ys
diff --git a/src/Data/List/Relation/Binary/Permutation/Algorithmic/Properties.agda b/src/Data/List/Relation/Binary/Permutation/Algorithmic/Properties.agda
new file mode 100644
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+++ b/src/Data/List/Relation/Binary/Permutation/Algorithmic/Properties.agda
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+-------------------------------------------------------------------------------
+-- The Agda standard library
+--
+-- Properties of the `Algorithmic` definition of the permutation relation.
+-------------------------------------------------------------------------------
+
+{-# OPTIONS --cubical-compatible --safe #-}
+
+open import Relation.Binary.Bundles using (Setoid)
+
+module Data.List.Relation.Binary.Permutation.Algorithmic.Properties
+  {s ℓ} (S : Setoid s ℓ) where
+
+open import Data.List.Base using (List; []; _∷_; _++_)
+open import Data.List.Properties using (++-identityʳ)
+import Relation.Binary.PropositionalEquality as ≡
+  using (sym)
+
+open import Data.List.Relation.Binary.Equality.Setoid S as ≋
+  using (≋-reflexive)
+open import Data.List.Relation.Binary.Permutation.Algorithmic S
+open import Data.List.Relation.Binary.Permutation.Setoid S as ↭
+  using (_↭_; ↭-refl; ↭-prep; ↭-swap; ↭-trans)
+
+open Setoid S
+  renaming (Carrier to A; refl to ≈-refl; sym to ≈-sym; trans to ≈-trans)
+
+private
+  variable
+    a b c d : A
+    as bs cs ds : List A
+
+
+-------------------------------------------------------------------------------
+-- Properties
+
+∼-swap : a ≈ c → b ≈ d → cs ∼ ds → a ∷ b ∷ cs ∼ d ∷ c ∷ ds
+∼-swap a≈c b≈d cs≈ds = (b≈d ∷ cs≈ds) ⋎ (a≈c ∷ ∼-refl _)
+
+∼-swap-++ : (as bs : List A) → as ++ bs ∼ bs ++ as
+∼-swap-++ [] bs = ∼-reflexive (≋-reflexive (≡.sym (++-identityʳ bs)))
+∼-swap-++ (a ∷ as) bs = lemma bs (∼-swap-++ as bs)
+  where
+  lemma : ∀ bs → cs ∼ bs ++ as → a ∷ cs ∼ bs ++ a ∷ as
+  lemma []        cs∼as
+    = a ≡∷ cs∼as
+  lemma (b ∷ bs) (a≈b ∷ cs∼bs++as)
+    = (a≈b ∷ ∼-refl _) ⋎ lemma bs cs∼bs++as
+  lemma (b ∷ bs) (cs∼b∷ds ⋎ a∷ds∼bs++as)
+    = (cs∼b∷ds ⋎ (∼-refl _)) ⋎ (lemma bs a∷ds∼bs++as)
+
+∼-congʳ : ∀ cs → as ∼ bs → cs ++ as ∼ cs ++ bs
+∼-congʳ {as = as} {bs = bs} cs as∼bs = lemma cs
+  where
+  lemma : ∀ cs → cs ++ as ∼ cs ++ bs
+  lemma []       = as∼bs
+  lemma (c ∷ cs) = c ≡∷ lemma cs
+
+∼-congˡ : as ∼ bs → ∀ cs → as ++ cs ∼ bs ++ cs
+∼-congˡ as∼bs cs = lemma as∼bs
+  where
+  lemma : as ∼ bs → as ++ cs ∼ bs ++ cs
+  lemma []                  = ∼-refl cs
+  lemma (a≈b ∷ as∼bs)       = a≈b ∷ lemma as∼bs
+  lemma (as∼b∷xs ⋎ bs∼a∷xs) = lemma as∼b∷xs ⋎ lemma bs∼a∷xs
+
+∼-cong : as ∼ bs → cs ∼ ds → as ++ cs ∼ bs ++ ds
+∼-cong as∼bs cs∼ds = ∼-trans (∼-congˡ as∼bs _) (∼-congʳ _ cs∼ds)
+
+-------------------------------------------------------------------------------
+-- Equivalence with `Setoid` definition _↭_
+
+↭⇒∼ : as ↭ bs → as ∼ bs
+↭⇒∼ (↭.refl as≋bs)         = ∼-reflexive as≋bs
+↭⇒∼ (↭.prep a≈b as∼bs)     = a≈b ∷ ↭⇒∼ as∼bs
+↭⇒∼ (↭.swap a≈c b≈d cs∼ds) = ∼-swap a≈c b≈d (↭⇒∼ cs∼ds)
+↭⇒∼ (↭.trans as∼bs bs∼cs)  = ∼-trans (↭⇒∼ as∼bs) (↭⇒∼ bs∼cs)
+
+∼⇒↭ : as ∼ bs → as ↭ bs
+∼⇒↭ []                  = ↭.↭-refl
+∼⇒↭ (a≈b ∷ as∼bs)       = ↭.prep a≈b (∼⇒↭ as∼bs)
+∼⇒↭ (as∼b∷cs ⋎ a∷cs∼bs) = ↭-trans (↭-prep _ (∼⇒↭ as∼b∷cs))
+                            (↭-trans (↭-swap _ _ ↭-refl)
+                               (↭-prep _ (∼⇒↭ a∷cs∼bs)))
diff --git a/src/Data/List/Relation/Binary/Permutation/Declarative.agda b/src/Data/List/Relation/Binary/Permutation/Declarative.agda
new file mode 100644
index 0000000000..757eea9761
--- /dev/null
+++ b/src/Data/List/Relation/Binary/Permutation/Declarative.agda
@@ -0,0 +1,104 @@
+-------------------------------------------------------------------------------
+-- The Agda standard library
+--
+-- A declarative definition of the permutation relation,
+-- as the least congruence making `_++_` commutative
+-------------------------------------------------------------------------------
+
+{-# OPTIONS --cubical-compatible --safe #-}
+
+open import Relation.Binary.Bundles using (Setoid)
+
+module Data.List.Relation.Binary.Permutation.Declarative
+  {s ℓ} (S : Setoid s ℓ) where
+
+open import Data.List.Base using (List; []; _∷_; [_]; _++_)
+open import Data.List.Properties using (++-identityʳ)
+open import Function.Base using (id; _∘_)
+open import Level using (_⊔_)
+import Relation.Binary.PropositionalEquality as ≡ using (sym)
+
+open import Data.List.Relation.Binary.Equality.Setoid S as ≋
+  using (_≋_; []; _∷_; ≋-refl; ≋-reflexive)
+
+open Setoid S
+  renaming (Carrier to A; refl to ≈-refl; sym to ≈-sym; trans to ≈-trans)
+
+private
+  variable
+    a b c d : A
+    as bs cs ds : List A
+
+
+-------------------------------------------------------------------------------
+-- Definition
+
+infix  4  _∼_
+
+data _∼_ : List A → List A → Set (s ⊔ ℓ) where
+  []     : [] ∼ []
+  _∷_    : a ≈ b → as ∼ bs → a ∷ as ∼ b ∷ bs
+  trans  : as ∼ bs → bs ∼ cs → as ∼ cs
+  _++ᵒ_  : ∀ as bs → as ++ bs ∼ bs ++ as
+
+-- smart constructor for prefix congruence
+
+_≡∷_  : ∀ c → as ∼ bs → c ∷ as ∼ c ∷ bs
+_≡∷_ c = ≈-refl ∷_
+
+-------------------------------------------------------------------------------
+-- Basic properties and smart constructors
+
+∼-reflexive : as ≋ bs → as ∼ bs
+∼-reflexive []            = []
+∼-reflexive (a≈b ∷ as∼bs) = a≈b ∷ ∼-reflexive as∼bs
+
+∼-refl : ∀ as → as ∼ as
+∼-refl _ = ∼-reflexive ≋-refl
+
+∼-sym : as ∼ bs → bs ∼ as
+∼-sym []                  = []
+∼-sym (a≈b ∷ as∼bs)       = ≈-sym a≈b ∷ ∼-sym as∼bs
+∼-sym (trans as∼cs cs∼bs) = trans (∼-sym cs∼bs) (∼-sym as∼cs)
+∼-sym (as ++ᵒ bs)         = bs ++ᵒ as
+
+-- smart constructor for trans
+
+∼-trans  : as ∼ bs → bs ∼ cs → as ∼ cs
+∼-trans []                  = id
+∼-trans (trans as∼bs bs∼cs) = ∼-trans as∼bs ∘ ∼-trans bs∼cs
+∼-trans as∼bs               = trans as∼bs
+
+-- smart constructor for swap
+
+∼-swap-++ : (as bs : List A) → as ++ bs ∼ bs ++ as
+∼-swap-++ []         bs         = ∼-reflexive (≋-reflexive (≡.sym (++-identityʳ bs)))
+∼-swap-++ as@(_ ∷ _) []         = ∼-reflexive (≋-reflexive (++-identityʳ as))
+∼-swap-++ as@(_ ∷ _) bs@(_ ∷ _) = as ++ᵒ bs
+
+∼-congʳ : as ∼ bs → cs ++ as ∼ cs ++ bs
+∼-congʳ {as = as} {bs = bs} {cs = cs} as∼bs = lemma cs
+  where
+  lemma : ∀ cs → cs ++ as ∼ cs ++ bs
+  lemma []       = as∼bs
+  lemma (c ∷ cs) = c ≡∷ lemma cs
+
+∼-congˡ : as ∼ bs → as ++ cs ∼ bs ++ cs
+∼-congˡ {as = as} {bs = bs} {cs = cs} as∼bs =
+  ∼-trans (∼-swap-++ as cs) (∼-trans (∼-congʳ as∼bs) (∼-swap-++ cs bs))
+
+∼-cong : as ∼ bs → cs ∼ ds → as ++ cs ∼ bs ++ ds
+∼-cong as∼bs cs∼ds = ∼-trans (∼-congˡ as∼bs) (∼-congʳ cs∼ds)
+
+-- smart constructor for generalised swap
+
+infix  5 _∼-⋎_
+
+_∼-⋎_ : as ∼ b ∷ cs → a ∷ cs ∼ bs → a ∷ as ∼ b ∷ bs
+_∼-⋎_ {b = b} {a = a} as∼b∷cs a∷cs∼bs =
+  trans (a ≡∷ as∼b∷cs) (∼-trans (∼-congˡ ([ a ] ++ᵒ [ b ])) (b ≡∷ a∷cs∼bs))
+
+⋎-syntax : ∀ cs → as ∼ b ∷ cs → a ∷ cs ∼ bs → a ∷ as ∼ b ∷ bs
+⋎-syntax cs = _∼-⋎_ {cs = cs}
+
+syntax ⋎-syntax cs as∼b∷cs a∷cs∼bs = as∼b∷cs ∼-⋎[ cs ] a∷cs∼bs
diff --git a/src/Data/List/Relation/Binary/Permutation/Declarative/Properties.agda b/src/Data/List/Relation/Binary/Permutation/Declarative/Properties.agda
new file mode 100644
index 0000000000..962581e821
--- /dev/null
+++ b/src/Data/List/Relation/Binary/Permutation/Declarative/Properties.agda
@@ -0,0 +1,65 @@
+-------------------------------------------------------------------------------
+-- The Agda standard library
+--
+-- Properties of declarative definition of permutation
+-------------------------------------------------------------------------------
+
+{-# OPTIONS --cubical-compatible --safe #-}
+
+open import Relation.Binary.Bundles using (Setoid)
+
+module Data.List.Relation.Binary.Permutation.Declarative.Properties
+  {s ℓ} (S : Setoid s ℓ) where
+
+open import Data.List.Base using (List; []; _∷_; length; _++_)
+open import Data.List.Properties using (length-++)
+open import Data.Nat.Base using (suc; _+_)
+open import Data.Nat.Properties using (+-comm)
+open import Relation.Binary.PropositionalEquality as ≡ using (_≡_)
+open import Relation.Binary.PropositionalEquality.Properties as ≡
+  using (module ≡-Reasoning)
+
+open import Data.List.Relation.Binary.Permutation.Algorithmic S
+  using ([]; _∷_; _⋎_)
+  renaming (_∼_ to _∼ₐ_; ∼-trans to ∼ₐ-trans)
+open import Data.List.Relation.Binary.Permutation.Algorithmic.Properties S
+  using ()
+  renaming (∼-swap-++ to ∼ₐ-swap-++)
+open import Data.List.Relation.Binary.Permutation.Declarative S
+
+open Setoid S
+  using ()
+  renaming (Carrier to A)
+
+private
+  variable
+    as bs : List A
+
+
+-------------------------------------------------------------------------------
+-- Properties
+
+∼-length : as ∼ bs → length as ≡ length bs
+∼-length []                  = ≡.refl
+∼-length (a≈b ∷ as∼bs)       = ≡.cong suc (∼-length as∼bs)
+∼-length (trans as∼cs cs∼bs) = ≡.trans (∼-length as∼cs) (∼-length cs∼bs)
+∼-length (as ++ᵒ bs)         = begin
+  length (as ++ bs)     ≡⟨ length-++ as ⟩
+  length as + length bs ≡⟨ +-comm (length as) (length bs) ⟩
+  length bs + length as ≡⟨ length-++ bs ⟨
+  length (bs ++ as)     ∎
+  where open ≡-Reasoning
+
+-------------------------------------------------------------------------------
+-- Equivalence with `Algorithmic` definition of _∼_
+
+∼ₐ⇒∼ : as ∼ₐ bs → as ∼ bs
+∼ₐ⇒∼ []                  = []
+∼ₐ⇒∼ (a≈b ∷ as∼bs)       = a≈b ∷ ∼ₐ⇒∼ as∼bs
+∼ₐ⇒∼ (as∼b∷cs ⋎ a∷cs∼bs) = ∼ₐ⇒∼ as∼b∷cs ∼-⋎ ∼ₐ⇒∼ a∷cs∼bs
+
+∼⇒∼ₐ : as ∼ bs → as ∼ₐ bs
+∼⇒∼ₐ []                  = []
+∼⇒∼ₐ (a≈b ∷ as∼bs)       = a≈b ∷ ∼⇒∼ₐ as∼bs
+∼⇒∼ₐ (trans as∼cs cs∼bs) = ∼ₐ-trans (∼⇒∼ₐ as∼cs) (∼⇒∼ₐ cs∼bs)
+∼⇒∼ₐ (as ++ᵒ bs)         = ∼ₐ-swap-++ as bs