Theorem: TensorPower.mul_assoc
File: Mathlib/LinearAlgebra/TensorPower.lean

tactic: let mul : ∀ n m : ℕ, ⨂[R]^n M →ₗ[R] (⨂[R]^m) M →ₗ[R] (⨂[R]^(n + m)) M := fun n m =>
  (TensorProduct.mk R _ _).compr₂ ↑(mulEquiv : _ ≃ₗ[R] (⨂[R]^(n + m)) M)
state_before: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
state_after: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)

tactic: let e : (⨂[R]^(na + nb + nc)) M ≃ₗ[R] (⨂[R]^(na + (nb + nc))) M := cast R M (add_assoc _ _ _)
state_before: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
state_after: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)

tactic: let lhs : (⨂[R]^na) M →ₗ[R] (⨂[R]^nb) M →ₗ[R] (⨂[R]^nc) M →ₗ[R] (⨂[R]^(na + (nb + nc))) M :=
  (LinearMap.llcomp R _ _ _ ((mul _ nc).compr₂ e.toLinearMap)).comp (mul na nb)
state_before: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
state_after: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)

tactic: have lhs_eq : ∀ a b c, lhs a b c = e (a ₜ* b ₜ* c) := fun _ _ _ => rfl
state_before: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
state_after: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)

tactic: let rhs : (⨂[R]^na) M →ₗ[R] (⨂[R]^nb) M →ₗ[R] (⨂[R]^nc) M →ₗ[R] (⨂[R]^(na + (nb + nc))) M :=
  (LinearMap.llcomp R _ _ _ (LinearMap.lflip (R := R)) <|
      (LinearMap.llcomp R _ _ _ (mul na _).flip).comp (mul nb nc)).flip
state_before: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
state_after: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)

tactic: have rhs_eq : ∀ a b c, rhs a b c = a ₜ* (b ₜ* c) := fun _ _ _ => rfl
state_before: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
state_after: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)

tactic: suffices lhs = rhs from
  LinearMap.congr_fun (LinearMap.congr_fun (LinearMap.congr_fun this a) b) c
state_before: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
⊢ (cast R M ⋯) (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c) =
    GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
state_after: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
⊢ lhs = rhs

tactic: ext a b c
state_before: R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a : ⨂[R]^na M
b : ⨂[R]^nb M
c : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
⊢ lhs = rhs
state_after: case H.H.H.H.H.H
R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a✝ : ⨂[R]^na M
b✝ : ⨂[R]^nb M
c✝ : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
a : Fin na → M
b : Fin nb → M
c : Fin nc → M
⊢ (((((lhs.compMultilinearMap (tprod R)) a).compMultilinearMap (tprod R)) b).compMultilinearMap (tprod R)) c =
    (((((rhs.compMultilinearMap (tprod R)) a).compMultilinearMap (tprod R)) b).compMultilinearMap (tprod R)) c

tactic: simp only [e, LinearMap.compMultilinearMap_apply, lhs_eq, rhs_eq, tprod_mul_tprod, cast_tprod]
state_before: case H.H.H.H.H.H
R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a✝ : ⨂[R]^na M
b✝ : ⨂[R]^nb M
c✝ : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
a : Fin na → M
b : Fin nb → M
c : Fin nc → M
⊢ (((((lhs.compMultilinearMap (tprod R)) a).compMultilinearMap (tprod R)) b).compMultilinearMap (tprod R)) c =
    (((((rhs.compMultilinearMap (tprod R)) a).compMultilinearMap (tprod R)) b).compMultilinearMap (tprod R)) c
state_after: case H.H.H.H.H.H
R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a✝ : ⨂[R]^na M
b✝ : ⨂[R]^nb M
c✝ : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
a : Fin na → M
b : Fin nb → M
c : Fin nc → M
⊢ (tprod R) (Fin.append (Fin.append a b) c ∘ Fin.cast ⋯) = (tprod R) (Fin.append a (Fin.append b c))

tactic: congr with j
state_before: case H.H.H.H.H.H
R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a✝ : ⨂[R]^na M
b✝ : ⨂[R]^nb M
c✝ : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
a : Fin na → M
b : Fin nb → M
c : Fin nc → M
⊢ (tprod R) (Fin.append (Fin.append a b) c ∘ Fin.cast ⋯) = (tprod R) (Fin.append a (Fin.append b c))
state_after: case H.H.H.H.H.H.h.e_6.h.h
R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a✝ : ⨂[R]^na M
b✝ : ⨂[R]^nb M
c✝ : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
a : Fin na → M
b : Fin nb → M
c : Fin nc → M
j : Fin (na + (nb + nc))
⊢ (Fin.append (Fin.append a b) c ∘ Fin.cast ⋯) j = Fin.append a (Fin.append b c) j

tactic: rw [Fin.append_assoc]
state_before: case H.H.H.H.H.H.h.e_6.h.h
R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a✝ : ⨂[R]^na M
b✝ : ⨂[R]^nb M
c✝ : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
a : Fin na → M
b : Fin nb → M
c : Fin nc → M
j : Fin (na + (nb + nc))
⊢ (Fin.append (Fin.append a b) c ∘ Fin.cast ⋯) j = Fin.append a (Fin.append b c) j
state_after: case H.H.H.H.H.H.h.e_6.h.h
R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a✝ : ⨂[R]^na M
b✝ : ⨂[R]^nb M
c✝ : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
a : Fin na → M
b : Fin nb → M
c : Fin nc → M
j : Fin (na + (nb + nc))
⊢ ((Fin.append a (Fin.append b c) ∘ Fin.cast ⋯) ∘ Fin.cast ⋯) j = Fin.append a (Fin.append b c) j

tactic: refine congr_arg (Fin.append a (Fin.append b c)) (Fin.ext ?_)
state_before: case H.H.H.H.H.H.h.e_6.h.h
R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a✝ : ⨂[R]^na M
b✝ : ⨂[R]^nb M
c✝ : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
a : Fin na → M
b : Fin nb → M
c : Fin nc → M
j : Fin (na + (nb + nc))
⊢ ((Fin.append a (Fin.append b c) ∘ Fin.cast ⋯) ∘ Fin.cast ⋯) j = Fin.append a (Fin.append b c) j
state_after: case H.H.H.H.H.H.h.e_6.h.h
R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a✝ : ⨂[R]^na M
b✝ : ⨂[R]^nb M
c✝ : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
a : Fin na → M
b : Fin nb → M
c : Fin nc → M
j : Fin (na + (nb + nc))
⊢ ↑(Fin.cast ⋯ (Fin.cast ⋯ j)) = ↑j

tactic: rw [Fin.coe_cast, Fin.coe_cast]
state_before: case H.H.H.H.H.H.h.e_6.h.h
R : Type u_1
M : Type u_2
inst✝² : CommSemiring R
inst✝¹ : AddCommMonoid M
inst✝ : Module R M
na nb nc : ℕ
a✝ : ⨂[R]^na M
b✝ : ⨂[R]^nb M
c✝ : ⨂[R]^nc M
mul : (n m : ℕ) → ⨂[R]^n M →ₗ[R] ⨂[R]^m M →ₗ[R] ⨂[R]^(n + m) M :=
  fun n m => (TensorProduct.mk R (⨂[R]^n M) (⨂[R]^m M)).compr₂ ↑mulEquiv
e : ⨂[R]^(na + nb + nc) M ≃ₗ[R] ⨂[R]^(na + (nb + nc)) M := cast R M ⋯
lhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^(na + nb) M) (⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
      ((mul (na + nb) nc).compr₂ ↑e) ∘ₗ
    mul na nb
lhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M),
    ((lhs a) b) c = e (GradedMonoid.GMul.mul (GradedMonoid.GMul.mul a b) c)
rhs : ⨂[R]^na M →ₗ[R] ⨂[R]^nb M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M :=
  (((LinearMap.llcomp R (⨂[R]^nb M) (⨂[R]^nc M →ₗ[R] ⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M)
          (⨂[R]^na M →ₗ[R] ⨂[R]^nc M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
        LinearMap.lflip)
      ((LinearMap.llcomp R (⨂[R]^nc M) (⨂[R]^(nb + nc) M) (⨂[R]^na M →ₗ[R] ⨂[R]^(na + (nb + nc)) M))
          (mul na (nb + nc)).flip ∘ₗ
        mul nb nc)).flip
rhs_eq :
  ∀ (a : ⨂[R]^na M) (b : ⨂[R]^nb M) (c : ⨂[R]^nc M), ((rhs a) b) c = GradedMonoid.GMul.mul a (GradedMonoid.GMul.mul b c)
a : Fin na → M
b : Fin nb → M
c : Fin nc → M
j : Fin (na + (nb + nc))
⊢ ↑(Fin.cast ⋯ (Fin.cast ⋯ j)) = ↑j
state_after: no goals