-- Lean 4 proof step from Mathlib.LinearAlgebra.Alternating.DomCoprod (goal)
Goal:
ιa : Type u_1
ιb : Type u_2
inst : Fintype ιa
inst_1 : Fintype ιb
R' : Type u_3
Mᵢ : Type u_4
N₁ : Type u_5
N₂ : Type u_6
inst_2 : CommSemiring R'
inst_3 : AddCommGroup N₁
inst_4 : Module R' N₁
inst_5 : AddCommGroup N₂
inst_6 : Module R' N₂
inst_7 : AddCommMonoid Mᵢ
inst_8 : Module R' Mᵢ
inst_9 : DecidableEq ιa
inst_10 : DecidableEq ιb
m : Mᵢ [⋀^ιa]→ₗ[R'] N₁
n₁ n₂ : Mᵢ [⋀^ιb]→ₗ[R'] N₂
x : ιa ⊕ ιb → Mᵢ
a : Perm (ιa ⊕ ιb)
⊢ (Quotient.liftOn' (s₁ := QuotientGroup.leftRel (Perm.sumCongrHom ιa ιb).range) (Quotient.mk'' a)
          (fun (σ : Perm (ιa ⊕ ιb)) ↦
            HSMul.hSMul (α := ℤˣ) (((Perm.sign : Perm (ιa ⊕ ιb) →* ℤˣ) : (a : Perm (ιa ⊕ ιb)) → ℤˣ) σ : ℤˣ)
              (MultilinearMap.domDomCongr σ
                ((↑m : MultilinearMap R' (fun (x : ιa) ↦ Mᵢ) N₁).domCoprod
                  (↑(n₁ + n₂) : MultilinearMap R' (fun (x : ιb) ↦ Mᵢ) N₂))))
          ⋯ :
        (a : (i : ιa ⊕ ιb) → Mᵢ) → N₁ ⊗[R'] N₂)
      x =
    HAdd.hAdd (α := N₁ ⊗[R'] N₂) (β := N₁ ⊗[R'] N₂)
      ((Quotient.liftOn' (s₁ := QuotientGroup.leftRel (Perm.sumCongrHom ιa ιb).range) (Quotient.mk'' a)
              (fun (σ : Perm (ιa ⊕ ιb)) ↦
                HSMul.hSMul (α := ℤˣ) (((Perm.sign : Perm (ιa ⊕ ιb) →* ℤˣ) : (a : Perm (ιa ⊕ ιb)) → ℤˣ) σ : ℤˣ)
                  (MultilinearMap.domDomCongr σ
                    ((↑m : MultilinearMap R' (fun (x : ιa) ↦ Mᵢ) N₁).domCoprod
                      (↑n₁ : MultilinearMap R' (fun (x : ιb) ↦ Mᵢ) N₂))))
              ⋯ :
            (a : (i : ιa ⊕ ιb) → Mᵢ) → N₁ ⊗[R'] N₂)
          x :
        N₁ ⊗[R'] N₂)
      ((Quotient.liftOn' (s₁ := QuotientGroup.leftRel (Perm.sumCongrHom ιa ιb).range) (Quotient.mk'' a)
              (fun (σ : Perm (ιa ⊕ ιb)) ↦
                HSMul.hSMul (α := ℤˣ) (((Perm.sign : Perm (ιa ⊕ ιb) →* ℤˣ) : (a : Perm (ιa ⊕ ιb)) → ℤˣ) σ : ℤˣ)
                  (MultilinearMap.domDomCongr σ
                    ((↑m : MultilinearMap R' (fun (x : ιa) ↦ Mᵢ) N₁).domCoprod
                      (↑n₂ : MultilinearMap R' (fun (x : ιb) ↦ Mᵢ) N₂))))
              ⋯ :
            (a : (i : ιa ⊕ ιb) → Mᵢ) → N₁ ⊗[R'] N₂)
          x :
        N₁ ⊗[R'] N₂)
have refine_3 : HSMul.hSMul (α := ℤˣ) (β := N₁ ⊗[R'] N₂) (((Perm.sign : Perm (ιa ⊕ ιb) →* ℤˣ) : (a : Perm (ιa ⊕ ιb)) → ℤˣ) a : ℤˣ)
    ((MultilinearMap.domDomCongr a
            ((MultilinearMap.domCoprod' :
                (a : MultilinearMap R' (fun (x : ιa) ↦ Mᵢ) N₁ ⊗[R'] MultilinearMap R' (fun (x : ιb) ↦ Mᵢ) N₂) →
                  MultilinearMap R' (fun (x : ιa ⊕ ιb) ↦ Mᵢ) (N₁ ⊗[R'] N₂))
              ((↑m : MultilinearMap R' (fun (x : ιa) ↦ Mᵢ) N₁) ⊗ₜ[R']
                ((↑n₁ : MultilinearMap R' (fun (x : ιb) ↦ Mᵢ) N₂) +
                  (↑n₂ : MultilinearMap R' (fun (x : ιb) ↦ Mᵢ) N₂)))) :
          (a : (i : ιa ⊕ ιb) → Mᵢ) → N₁ ⊗[R'] N₂)
        x :
      N₁ ⊗[R'] N₂) =
  HSMul.hSMul (α := ℤˣ) (β := N₁ ⊗[R'] N₂) (((Perm.sign : Perm (ιa ⊕ ιb) →* ℤˣ) : (a : Perm (ιa ⊕ ιb)) → ℤˣ) a : ℤˣ)
      ((MultilinearMap.domDomCongr a
              ((MultilinearMap.domCoprod' :
                  (a : MultilinearMap R' (fun (x : ιa) ↦ Mᵢ) N₁ ⊗[R'] MultilinearMap R' (fun (x : ιb) ↦ Mᵢ) N₂) →
                    MultilinearMap R' (fun (x : ιa ⊕ ιb) ↦ Mᵢ) (N₁ ⊗[R'] N₂))
                ((↑m : MultilinearMap R' (fun (x : ιa) ↦ Mᵢ) N₁) ⊗ₜ[R']
                  (↑n₁ : MultilinearMap R' (fun (x : ιb) ↦ Mᵢ) N₂))) :
            (a : (i : ιa ⊕ ιb) → Mᵢ) → N₁ ⊗[R'] N₂)
          x :
        N₁ ⊗[R'] N₂) +
    HSMul.hSMul (α := ℤˣ) (β := N₁ ⊗[R'] N₂) (((Perm.sign : Perm (ιa ⊕ ιb) →* ℤˣ) : (a : Perm (ιa ⊕ ιb)) → ℤˣ) a : ℤˣ)
      ((MultilinearMap.domDomCongr a
              ((MultilinearMap.domCoprod' :
                  (a : MultilinearMap R' (fun (x : ιa) ↦ Mᵢ) N₁ ⊗[R'] MultilinearMap R' (fun (x : ιb) ↦ Mᵢ) N₂) →
                    MultilinearMap R' (fun (x : ιa ⊕ ιb) ↦ Mᵢ) (N₁ ⊗[R'] N₂))
                ((↑m : MultilinearMap R' (fun (x : ιa) ↦ Mᵢ) N₁) ⊗ₜ[R']
                  (↑n₂ : MultilinearMap R' (fun (x : ιb) ↦ Mᵢ) N₂))) :
            (a : (i : ιa ⊕ ιb) → Mᵢ) → N₁ ⊗[R'] N₂)
          x :
        N₁ ⊗[R'] N₂)
Tactic: simp only [Quotient.liftOn'_mk'', coe_add, coe_smul, MultilinearMap.smul_apply,
        ← MultilinearMap.domCoprod'_apply]