Module: Mathlib.CategoryTheory.Abelian.Projective.Ext

Goal state before:
pp: C : Type u
inst✝² : Category.{v, u} C
inst✝¹ : Abelian C
inst✝ : HasExt C
X Y : C
R : ProjectiveResolution X
n : ℕ
f : R.complex.X n ⟶ Y
m : ℕ
hm : n + 1 = m
hf : R.complex.d m n ≫ f = 0
X' : C
R' : ProjectiveResolution X'
g : X' ⟶ X
φ : R'.Hom R g
this✝ : HasDerivedCategory C
this : (R'.cochainComplexXIso (-↑n) n ⋯).hom ≫ φ.hom.f n = φ.hom'.f (-↑n) ≫ (R.cochainComplexXIso (-↑n) n ⋯).hom
⊢ (ShiftedHom.mk₀ (↑0) Ext.mk₀._proof_1 ((DerivedCategory.singleFunctor C 0).map g)).comp
      ((ShiftedHom.mk₀ 0 ⋯
            ((DerivedCategory.singleFunctorIsoCompQ C 0).hom.app X ≫ inv (DerivedCategory.Q.map R.π'))).comp
        ((ShiftedHom.map
              (Cocycle.equivHomShift.symm (Cocycle.toSingleMk ((R.cochainComplexXIso (-↑n) n ⋯).hom ≫ f) ⋯ (-↑m) ⋯ ⋯))
              DerivedCategory.Q).comp
          (ShiftedHom.mk₀ 0 ⋯ ((DerivedCategory.singleFunctorIsoCompQ C 0).inv.app Y)) ⋯)
        ⋯)
      ⋯ =
    (ShiftedHom.mk₀ 0 ⋯
          ((DerivedCategory.singleFunctorIsoCompQ C 0).hom.app X' ≫ inv (DerivedCategory.Q.map R'.π'))).comp
      ((ShiftedHom.map
            (Cocycle.equivHomShift.symm
              (Cocycle.toSingleMk (φ.hom'.f (-↑n) ≫ (R.cochainComplexXIso (-↑n) n ⋯).hom ≫ f) ⋯ (-↑m) ⋯ ⋯))
            DerivedCategory.Q).comp
        (ShiftedHom.mk₀ 0 ⋯ ((DerivedCategory.singleFunctorIsoCompQ C 0).inv.app Y)) ⋯)
      ⋯

Tactic:
rw [Cocycle.toSingleMk_precomp _ _ _ (by lia)
    (by simpa [R.cochainComplex_d _ _ _ _ rfl rfl]),
    Cocycle.equivHomShift_symm_precomp,
    ← ShiftedHom.mk₀_comp 0 rfl, ShiftedHom.map_comp,
    ← ShiftedHom.comp_assoc _ _ _ (zero_add _) _ (by simp),
    ← ShiftedHom.comp_assoc _ _ _ (add_zero _) _ (by simp),
    ← ShiftedHom.comp_assoc _ _ _ (add_zero _) _ (by simp),
    ← ShiftedHom.comp_assoc _ _ _ (zero_add _) _ (by simp),
    ShiftedHom.map_mk₀, ShiftedHom.mk₀_comp_mk₀, ShiftedHom.mk₀_comp_mk₀]

Goal state after:
C : Type u
inst✝² : Category.{v, u} C
inst✝¹ : Abelian C
inst✝ : HasExt C
X Y : C
R : ProjectiveResolution X
n : ℕ
f : R.complex.X n ⟶ Y
m : ℕ
hm : n + 1 = m
hf : R.complex.d m n ≫ f = 0
X' : C
R' : ProjectiveResolution X'
g : X' ⟶ X
φ : R'.Hom R g
this✝ : HasDerivedCategory C
this : (R'.cochainComplexXIso (-↑n) n ⋯).hom ≫ φ.hom.f n = φ.hom'.f (-↑n) ≫ (R.cochainComplexXIso (-↑n) n ⋯).hom
⊢ ((ShiftedHom.mk₀ ↑0 ⋯
              ((DerivedCategory.singleFunctor C 0).map g ≫
                (DerivedCategory.singleFunctorIsoCompQ C 0).hom.app X ≫ inv (DerivedCategory.Q.map R.π'))).comp
          (ShiftedHom.map
            (Cocycle.equivHomShift.symm (Cocycle.toSingleMk ((R.cochainComplexXIso (-↑n) n ⋯).hom ≫ f) ⋯ (-↑m) ⋯ ⋯))
            DerivedCategory.Q)
          ⋯).comp
      (ShiftedHom.mk₀ 0 ⋯ ((DerivedCategory.singleFunctorIsoCompQ C 0).inv.app Y)) ⋯ =
    ((ShiftedHom.mk₀ 0 ⋯
              (((DerivedCategory.singleFunctorIsoCompQ C 0).hom.app X' ≫ inv (DerivedCategory.Q.map R'.π')) ≫
                DerivedCategory.Q.map φ.hom')).comp
          (ShiftedHom.map
            (Cocycle.equivHomShift.symm (Cocycle.toSingleMk ((R.cochainComplexXIso (-↑n) n ⋯).hom ≫ f) ⋯ (-↑m) ⋯ ⋯))
            DerivedCategory.Q)
          ⋯).comp
      (ShiftedHom.mk₀ 0 ⋯ ((DerivedCategory.singleFunctorIsoCompQ C 0).inv.app Y)) ⋯