-- Mathlib/Algebra/Homology/Embedding/TruncGEHomology.lean
lemma quasiIsoAt_πTruncGE {j : ι} {j' : ι'} (hj' : e.f j = j') :
    QuasiIsoAt (K.πTruncGE e) j'

Proof state:
case neg
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
⊢ ShortComplex.QuasiIso ((shortComplexFunctor C c' j').map (K.πTruncGE e))

Tactic:
let φ := (shortComplexFunctor C c' j').map (K.πTruncGE e)

Result:
case neg
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
φ : (shortComplexFunctor C c' j').obj K ⟶ (shortComplexFunctor C c' j').obj (K.truncGE e) :=
  (shortComplexFunctor C c' j').map (K.πTruncGE e)
⊢ ShortComplex.QuasiIso ((shortComplexFunctor C c' j').map (K.πTruncGE e))

Proof state:
case neg
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
φ : (shortComplexFunctor C c' j').obj K ⟶ (shortComplexFunctor C c' j').obj (K.truncGE e) :=
  (shortComplexFunctor C c' j').map (K.πTruncGE e)
this✝ : Epi φ.τ₁
this : IsIso φ.τ₂
⊢ ShortComplex.QuasiIso ((shortComplexFunctor C c' j').map (K.πTruncGE e))

Tactic:
have : IsIso φ.τ₃ := by
  dsimp [φ, πTruncGE]
  have : c'.next j' = e.f (c.next j) := by simpa only [← hj'] using e.next_f rfl
  rw [e.isIso_liftExtend_f_iff _ _ this.symm]
  exact K.isIso_restrictionToTruncGE' e _ (e.not_boundaryGE_next' hj rfl)

Result:
case neg
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
φ : (shortComplexFunctor C c' j').obj K ⟶ (shortComplexFunctor C c' j').obj (K.truncGE e) :=
  (shortComplexFunctor C c' j').map (K.πTruncGE e)
this✝¹ : Epi φ.τ₁
this✝ : IsIso φ.τ₂
this : IsIso φ.τ₃
⊢ ShortComplex.QuasiIso ((shortComplexFunctor C c' j').map (K.πTruncGE e))

Proof state:
case pos
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
φ : (shortComplexFunctor C c' j').obj K ⟶ (shortComplexFunctor C c' j').obj (K.truncGE e) :=
  (shortComplexFunctor C c' j').map (K.πTruncGE e)
hi : ∃ i, e.f i = c'.prev j'
⊢ Epi φ.τ₁

Tactic:
obtain ⟨i, hi⟩ := hi

Result:
case pos.intro
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
φ : (shortComplexFunctor C c' j').obj K ⟶ (shortComplexFunctor C c' j').obj (K.truncGE e) :=
  (shortComplexFunctor C c' j').map (K.πTruncGE e)
i : ι
hi : e.f i = c'.prev j'
⊢ Epi φ.τ₁

Proof state:
case neg
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
φ : (shortComplexFunctor C c' j').obj K ⟶ (shortComplexFunctor C c' j').obj (K.truncGE e) :=
  (shortComplexFunctor C c' j').map (K.πTruncGE e)
this : Epi φ.τ₁
⊢ ShortComplex.QuasiIso ((shortComplexFunctor C c' j').map (K.πTruncGE e))

Tactic:
have : IsIso φ.τ₂ := by
  dsimp [φ, πTruncGE]
  rw [e.isIso_liftExtend_f_iff _ _ hj']
  exact K.isIso_restrictionToTruncGE' e j hj

Result:
case neg
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
φ : (shortComplexFunctor C c' j').obj K ⟶ (shortComplexFunctor C c' j').obj (K.truncGE e) :=
  (shortComplexFunctor C c' j').map (K.πTruncGE e)
this✝ : Epi φ.τ₁
this : IsIso φ.τ₂
⊢ ShortComplex.QuasiIso ((shortComplexFunctor C c' j').map (K.πTruncGE e))

Proof state:
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
φ : (shortComplexFunctor C c' j').obj K ⟶ (shortComplexFunctor C c' j').obj (K.truncGE e) :=
  (shortComplexFunctor C c' j').map (K.πTruncGE e)
this✝ : Epi φ.τ₁
this : IsIso φ.τ₂
⊢ IsIso ((e.liftExtend (K.restrictionToTruncGE' e) ⋯).f (c'.next j'))

Tactic:
have : c'.next j' = e.f (c.next j) := by simpa only [← hj'] using e.next_f rfl

Result:
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
φ : (shortComplexFunctor C c' j').obj K ⟶ (shortComplexFunctor C c' j').obj (K.truncGE e) :=
  (shortComplexFunctor C c' j').map (K.πTruncGE e)
this✝¹ : Epi φ.τ₁
this✝ : IsIso φ.τ₂
this : c'.next j' = e.f (c.next j)
⊢ IsIso ((e.liftExtend (K.restrictionToTruncGE' e) ⋯).f (c'.next j'))

Proof state:
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
φ : (shortComplexFunctor C c' j').obj K ⟶ (shortComplexFunctor C c' j').obj (K.truncGE e) :=
  (shortComplexFunctor C c' j').map (K.πTruncGE e)
this : Epi φ.τ₁
⊢ IsIso ((K.restrictionToTruncGE' e).f j)

Tactic:
exact K.isIso_restrictionToTruncGE' e j hj

Result:
no goals

Proof state:
ι : Type u_1
ι' : Type u_2
c : ComplexShape ι
c' : ComplexShape ι'
C : Type u_3
inst✝⁴ : Category.{u_4, u_3} C
inst✝³ : HasZeroMorphisms C
K : HomologicalComplex C c'
e : c.Embedding c'
inst✝² : e.IsTruncGE
inst✝¹ : ∀ (i' : ι'), K.HasHomology i'
inst✝ : HasZeroObject C
j : ι
j' : ι'
hj' : e.f j = j'
hj : ¬e.BoundaryGE j
φ : (shortComplexFunctor C c' j').obj K ⟶ (shortComplexFunctor C c' j').obj (K.truncGE e) :=
  (shortComplexFunctor C c' j').map (K.πTruncGE e)
this✝¹ : Epi φ.τ₁
this✝ : IsIso φ.τ₂
this : c'.next j' = e.f (c.next j)
⊢ IsIso ((K.restrictionToTruncGE' e).f (c.next j))

Tactic:
exact K.isIso_restrictionToTruncGE' e _ (e.not_boundaryGE_next' hj rfl)

Result:
no goals