-- Lean 4 proof step from Mathlib.AlgebraicTopology.SimplicialSet.AnodyneExtensions.RelativeCellComplex (goal)
Goal:
X : SSet
A : X.Subcomplex
P : A.Pairing
ι : Type v
inst : LinearOrder ι
f : P.RankFunction ι
inst_1 : P.IsProper
inst_2 : SuccOrder ι
inst_3 : NoMaxOrder ι
j : ι
x : SimplexCategoryᵒᵖ
d : ℕ
x₁ x₂ : Functor.obj (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌) (f.sigmaStdSimplex j)
hx₁ :
  x₁ ∉
    Set.range (ι :=
      (fun (X : Type u) ↦ X) (Functor.obj (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌) (f.sigmaHorn j)))
      (⇑(ConcreteCategory.hom (FC := TypeCat.Fun)
            (CategoryTheory.Functor.map (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌) (f.m j))) :
        (a : (fun (X : Type u) ↦ X) (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌.obj (f.sigmaHorn j))) →
          (fun (X : Type u) ↦ X) (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌.obj (f.sigmaStdSimplex j)))
hx₂ :
  x₂ ∉
    Set.range (ι :=
      (fun (X : Type u) ↦ X) (Functor.obj (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌) (f.sigmaHorn j)))
      (⇑(ConcreteCategory.hom (FC := TypeCat.Fun)
            (CategoryTheory.Functor.map (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌) (f.m j))) :
        (a : (fun (X : Type u) ↦ X) (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌.obj (f.sigmaHorn j))) →
          (fun (X : Type u) ↦ X) (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌.obj (f.sigmaStdSimplex j)))
h :
  Eq (α :=
    (fun (X : Type u) ↦ X)
      (Functor.obj (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌) (f.filtration (Order.succ j)).toSSet))
    ((ConcreteCategory.hom (FC := TypeCat.Fun)
            (CategoryTheory.Functor.map (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌) (f.b j)) :
          (a :
              (fun (X : Type u) ↦ X)
                (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌.obj (f.sigmaStdSimplex j))) →
            (fun (X : Type u) ↦ X)
              (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌.obj (f.filtration (Order.succ j)).toSSet))
        x₁ :
      (fun (X : Type u) ↦ X)
        (Functor.obj (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌) (f.filtration (Order.succ j)).toSSet))
    ((ConcreteCategory.hom (FC := TypeCat.Fun)
            (CategoryTheory.Functor.map (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌) (f.b j)) :
          (a :
              (fun (X : Type u) ↦ X)
                (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌.obj (f.sigmaStdSimplex j))) →
            (fun (X : Type u) ↦ X)
              (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌.obj (f.filtration (Order.succ j)).toSSet))
        x₂ :
      (fun (X : Type u) ↦ X)
        (Functor.obj (CategoryTheory.evaluation SimplexCategoryᵒᵖ (Type u) _⦋d⦌) (f.filtration (Order.succ j)).toSSet))
s₁ : (range (X := f.sigmaHorn j) (f.m j)).N
g₁ : ⦋d⦌ ⟶ ⦋s₁.dim⦌
left : Epi g₁
hg₁ :
  (ConcreteCategory.hom (FC := TypeCat.Fun) ((f.sigmaStdSimplex j).map g₁.op) :
        (a : (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋s₁.dim⦌)) →
          (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋d⦌))
      s₁.simplex =
    x₁
s₂ : (range (X := f.sigmaHorn j) (f.m j)).N
g₂ : ⦋d⦌ ⟶ ⦋s₂.dim⦌
left_1 : Epi g₂
hg₂ :
  (ConcreteCategory.hom (FC := TypeCat.Fun) ((f.sigmaStdSimplex j).map g₂.op) :
        (a : (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋s₂.dim⦌)) →
          (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋d⦌))
      s₂.simplex =
    x₂
⊢ x₁ = x₂
have obtain : ∀ (g₂ : ⦋d⦌ ⟶ ⦋s₁.dim⦌),
  Epi g₂ →
    ∀
      (hg₂ :
        (ConcreteCategory.hom (FC := TypeCat.Fun) ((f.sigmaStdSimplex j).map g₂.op) :
              (a : (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋s₁.dim⦌)) →
                (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋d⦌))
            s₁.simplex =
          x₂),
      x₁ = x₂
Tactic: obtain rfl : s₁ = s₂ := f.isPushout_aux₃ (by
      dsimp
      rw [S.eq_iff_ofSimplex_eq, ← Subcomplex.ofSimplex_map_of_epi g₁,
        ← Subcomplex.ofSimplex_map_of_epi g₂]
      · simp [← dsimp% (f.b j).naturality_apply, hg₁, hg₂, dsimp% h]
      all_goals
      · rw [Subcomplex.mem_nonDegenerate_iff]
        apply f.isPushout_aux₁)
Cleared hypotheses: s₂, g₂, left_1, hg₂