-- Lean 4 proof step from Mathlib.RepresentationTheory.Homological.GroupHomology.Functoriality (hypothesis)
Goal:
k G : Type u
inst : CommRing k
inst_1 : Group G
A : Rep k G
S : Subgroup G
inst_2 : S.Normal
inst_3 : Representation.IsTrivial (MonoidHom.comp A.ρ S.subtype)
x : G →₀ (↑A : Type u)
hxc : x ∈ cycles₁ A
hx :
  Eq (α := (↑(H1 (A.ofQuotient S)) : Type u))
    (((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat k) ↦ (↑x1 : Type u) →ₗ[k] (↑x2 : Type u)) (CC :=
              ModuleCat.carrier) (X := ModuleCat.of k ↥(cycles₁ (A.ofQuotient S))) (Y :=
              groupHomology (A.ofQuotient S) 1) (H1π (A.ofQuotient S)) :
            (↑(ModuleCat.of k ↥(cycles₁ (A.ofQuotient S))) : Type u) →ₗ[k]
              (↑(groupHomology (A.ofQuotient S) 1) : Type u)) :
          (a : ↥(cycles₁ (A.ofQuotient S))) → (↑(groupHomology (A.ofQuotient S) 1) : Type u))
        ((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat k) ↦ (↑x1 : Type u) →ₗ[k] (↑x2 : Type u)) (CC :=
                ModuleCat.carrier) (Y := ModuleCat.of k ↥(cycles₁ (A.ofQuotient S)))
                (mapCycles₁ (QuotientGroup.mk' S) (Iso.inv (A.resOfQuotientIso S))) :
              (a : ↥(cycles₁ A)) → ↥(cycles₁ (A.ofQuotient S)))
            (Subtype.mk (p := fun (x : G →₀ (↑A : Type u)) ↦ x ∈ cycles₁ A) x hxc) :
          ↥(cycles₁ (A.ofQuotient S))) :
      (↑(groupHomology (A.ofQuotient S) 1) : Type u))
    (0 : (↑(H1 (A.ofQuotient S)) : Type u))
y : (↑(ModuleCat.of k ((G ⧸ S) × G ⧸ S →₀ (↑(A.ofQuotient S) : Type u))) : Type u)
hy :
  Eq (α := G ⧸ S →₀ (↑(A.ofQuotient S) : Type u))
    ((ModuleCat.Hom.hom (d₂₁ (A.ofQuotient S)) :
          (a : (↑(ModuleCat.of k ((G ⧸ S) × G ⧸ S →₀ (↑(A.ofQuotient S) : Type u))) : Type u)) →
            G ⧸ S →₀ (↑(A.ofQuotient S) : Type u))
        y :
      G ⧸ S →₀ (↑(A.ofQuotient S) : Type u))
    ((↑((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat k) ↦ (↑x1 : Type u) →ₗ[k] (↑x2 : Type u)) (CC :=
                ModuleCat.carrier) (Y := ModuleCat.of k ↥(cycles₁ (A.ofQuotient S)))
                (mapCycles₁ (QuotientGroup.mk' S) (Iso.inv (A.resOfQuotientIso S))) :
              (a : ↥(cycles₁ A)) → ↥(cycles₁ (A.ofQuotient S)))
            (Subtype.mk (p := fun (x : G →₀ (↑A : Type u)) ↦ x ∈ cycles₁ A) x hxc)) :
        G ⧸ S →₀ (↑(A.ofQuotient S) : Type u)) :
      G ⧸ S →₀ (↑(A.ofQuotient S) : Type u))
s : (b : G ⧸ S) → G
hs : ∀ (b : G ⧸ S), (QuotientGroup.mk' S : (a : G) → G ⧸ S) (s b) = b
hs₁ : QuotientGroup.mk ∘ s = id
z : G × G →₀ (↑A : Type u) :=
  (lmapDomain (↑A : Type u) k (Prod.map s s) : (a : (G ⧸ S) × G ⧸ S →₀ (↑A : Type u)) → G × G →₀ (↑A : Type u)) y
⊢ Eq (α := G ⧸ S →₀ (↑A : Type u))
    ((lmapDomain (↑A : Type u) k (⇑(QuotientGroup.mk' S) : (a : G) → G ⧸ S) :
          (a : G →₀ (↑A : Type u)) → G ⧸ S →₀ (↑A : Type u))
        ((ModuleCat.Hom.hom (d₂₁ A) :
            (a : (↑(ModuleCat.of k (G × G →₀ (↑A : Type u))) : Type u)) →
              (↑(ModuleCat.of k (G →₀ (↑A : Type u))) : Type u))
          z) :
      G ⧸ S →₀ (↑A : Type u))
    ((ModuleCat.Hom.hom (d₂₁ (A.ofQuotient S)) :
          (a : (↑(ModuleCat.of k ((G ⧸ S) × G ⧸ S →₀ (↑(A.ofQuotient S) : Type u))) : Type u)) →
            (↑(ModuleCat.of k (G ⧸ S →₀ (↑(A.ofQuotient S) : Type u))) : Type u))
        y :
      (↑(ModuleCat.of k (G ⧸ S →₀ (↑(A.ofQuotient S) : Type u))) : Type u))
have this : Eq (α := (↑(shortComplexH1 (A.ofQuotient S)).X₂ : Type u))
  ((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat k) ↦ (↑x1 : Type u) →ₗ[k] (↑x2 : Type u)) (CC :=
          ModuleCat.carrier) (Y := (shortComplexH1 (A.ofQuotient S)).X₂)
          ((shortComplexH1 A).f ≫ Hom.τ₂ (mapShortComplexH1 (QuotientGroup.mk' S) (Iso.inv (A.resOfQuotientIso S)))) :
        (a : (↑(shortComplexH1 A).X₁ : Type u)) → (↑(shortComplexH1 (A.ofQuotient S)).X₂ : Type u))
      z :
    (↑(shortComplexH1 (A.ofQuotient S)).X₂ : Type u))
  ((ConcreteCategory.hom (FC := fun (x1 x2 : ModuleCat k) ↦ (↑x1 : Type u) →ₗ[k] (↑x2 : Type u)) (CC :=
          ModuleCat.carrier)
          (Hom.τ₁ (mapShortComplexH1 (QuotientGroup.mk' S) (Iso.inv (A.resOfQuotientIso S))) ≫
            (shortComplexH1 (A.ofQuotient S)).f) :
        (a : (↑(shortComplexH1 A).X₁ : Type u)) → (↑(shortComplexH1 (A.ofQuotient S)).X₂ : Type u))
      z :
    (↑(shortComplexH1 (A.ofQuotient S)).X₂ : Type u))
Tactic: have := congr($((mapShortComplexH1 (QuotientGroup.mk' S)
      (resOfQuotientIso A S).inv).comm₁₂.symm) z)