-- 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
j : ι
x : f.Cell j
g : unop (op ⦋x.dim + 1⦌) ⟶ ⦋x.dim + 1⦌
hs_1 :
  Membership.mem (α := f.sigmaStdSimplex j _⦋x.dim + 1⦌) (γ := Set (f.sigmaStdSimplex j _⦋x.dim + 1⦌))
    ((f.sigmaStdSimplex j).nonDegenerate (x.dim + 1))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (x.ιSigmaStdSimplex.app (op ⦋x.dim + 1⦌)) :
          (a : (fun (X : Type u) ↦ X) (Δ[x.dim + 1] _⦋x.dim + 1⦌)) →
            (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌))
        ((Equiv.symm stdSimplex.objEquiv : (a : unop (op ⦋x.dim + 1⦌) ⟶ ⦋x.dim + 1⦌) → Δ[x.dim + 1] _⦋x.dim + 1⦌) g :
          Δ[x.dim + 1] _⦋x.dim + 1⦌) :
      (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌))
hs : Mono g
hs'_1 :
  ((ConcreteCategory.hom (FC := TypeCat.Fun) (x.ιSigmaStdSimplex.app (op ⦋x.dim + 1⦌)) :
          (a : (fun (X : Type u) ↦ X) (Δ[x.dim + 1] _⦋x.dim + 1⦌)) →
            (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌))
        ((Equiv.symm stdSimplex.objEquiv : (a : unop (op ⦋x.dim + 1⦌) ⟶ ⦋x.dim + 1⦌) → Δ[x.dim + 1] _⦋x.dim + 1⦌) g :
          Δ[x.dim + 1] _⦋x.dim + 1⦌) :
      (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌)) ∉
    (range (X := f.sigmaHorn j) (f.m j)).obj (op ⦋x.dim + 1⦌)
hs' :
  ((Equiv.symm stdSimplex.objEquiv : (a : unop (op ⦋x.dim + 1⦌) ⟶ ⦋x.dim + 1⦌) → Δ[x.dim + 1] _⦋x.dim + 1⦌) g :
      Δ[x.dim + 1] _⦋x.dim + 1⦌) ∉
    Λ[x.dim + 1, x.index].obj (op ⦋x.dim + 1⦌)
⊢ ∃ (c : f.Cell j),
    N.mk
          ((ConcreteCategory.hom (FC := TypeCat.Fun) (x.ιSigmaStdSimplex.app (op ⦋x.dim + 1⦌)) :
              (a : (fun (X : Type u) ↦ X) (Δ[x.dim + 1] _⦋x.dim + 1⦌)) →
                (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌))
            ((Equiv.symm stdSimplex.objEquiv : (a : unop (op ⦋x.dim + 1⦌) ⟶ ⦋x.dim + 1⦌) → Δ[x.dim + 1] _⦋x.dim + 1⦌)
                g :
              Δ[x.dim + 1] _⦋x.dim + 1⦌))
          hs_1 hs'_1 =
        Cell.type₁ f c ∨
      N.mk
          ((ConcreteCategory.hom (FC := TypeCat.Fun) (x.ιSigmaStdSimplex.app (op ⦋x.dim + 1⦌)) :
              (a : (fun (X : Type u) ↦ X) (Δ[x.dim + 1] _⦋x.dim + 1⦌)) →
                (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌))
            ((Equiv.symm stdSimplex.objEquiv : (a : unop (op ⦋x.dim + 1⦌) ⟶ ⦋x.dim + 1⦌) → Δ[x.dim + 1] _⦋x.dim + 1⦌)
                g :
              Δ[x.dim + 1] _⦋x.dim + 1⦌))
          hs_1 hs'_1 =
        Cell.type₂ f c
have inr : ∀
  (hs :
    Membership.mem (α := f.sigmaStdSimplex j _⦋x.dim + 1⦌) (γ := Set (f.sigmaStdSimplex j _⦋x.dim + 1⦌))
      ((f.sigmaStdSimplex j).nonDegenerate (x.dim + 1))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) (x.ιSigmaStdSimplex.app (op ⦋x.dim + 1⦌)) :
            (a : (fun (X : Type u) ↦ X) (Δ[x.dim + 1] _⦋x.dim + 1⦌)) →
              (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌))
          ((Equiv.symm stdSimplex.objEquiv : (a : unop (op ⦋x.dim + 1⦌) ⟶ ⦋x.dim + 1⦌) → Δ[x.dim + 1] _⦋x.dim + 1⦌)
              (𝟙 (unop (op ⦋x.dim + 1⦌))) :
            Δ[x.dim + 1] _⦋x.dim + 1⦌) :
        (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌)))
  (hs_2 : Mono (𝟙 (unop (op ⦋x.dim + 1⦌))))
  (hs' :
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (x.ιSigmaStdSimplex.app (op ⦋x.dim + 1⦌)) :
            (a : (fun (X : Type u) ↦ X) (Δ[x.dim + 1] _⦋x.dim + 1⦌)) →
              (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌))
          ((Equiv.symm stdSimplex.objEquiv : (a : unop (op ⦋x.dim + 1⦌) ⟶ ⦋x.dim + 1⦌) → Δ[x.dim + 1] _⦋x.dim + 1⦌)
              (𝟙 (unop (op ⦋x.dim + 1⦌))) :
            Δ[x.dim + 1] _⦋x.dim + 1⦌) :
        (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌)) ∉
      (range (X := f.sigmaHorn j) (f.m j)).obj (op ⦋x.dim + 1⦌))
  (hs'_2 :
    ((Equiv.symm stdSimplex.objEquiv : (a : unop (op ⦋x.dim + 1⦌) ⟶ ⦋x.dim + 1⦌) → Δ[x.dim + 1] _⦋x.dim + 1⦌)
          (𝟙 (unop (op ⦋x.dim + 1⦌))) :
        Δ[x.dim + 1] _⦋x.dim + 1⦌) ∉
      Λ[x.dim + 1, x.index].obj (op ⦋x.dim + 1⦌)),
  ∃ (c : f.Cell j),
    N.mk
          ((ConcreteCategory.hom (FC := TypeCat.Fun) (x.ιSigmaStdSimplex.app (op ⦋x.dim + 1⦌)) :
              (a : (fun (X : Type u) ↦ X) (Δ[x.dim + 1] _⦋x.dim + 1⦌)) →
                (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌))
            ((Equiv.symm stdSimplex.objEquiv : (a : unop (op ⦋x.dim + 1⦌) ⟶ ⦋x.dim + 1⦌) → Δ[x.dim + 1] _⦋x.dim + 1⦌)
                (𝟙 (unop (op ⦋x.dim + 1⦌))) :
              Δ[x.dim + 1] _⦋x.dim + 1⦌))
          hs hs' =
        Cell.type₁ f c ∨
      N.mk
          ((ConcreteCategory.hom (FC := TypeCat.Fun) (x.ιSigmaStdSimplex.app (op ⦋x.dim + 1⦌)) :
              (a : (fun (X : Type u) ↦ X) (Δ[x.dim + 1] _⦋x.dim + 1⦌)) →
                (fun (X : Type u) ↦ X) (f.sigmaStdSimplex j _⦋x.dim + 1⦌))
            ((Equiv.symm stdSimplex.objEquiv : (a : unop (op ⦋x.dim + 1⦌) ⟶ ⦋x.dim + 1⦌) → Δ[x.dim + 1] _⦋x.dim + 1⦌)
                (𝟙 (unop (op ⦋x.dim + 1⦌))) :
              Δ[x.dim + 1] _⦋x.dim + 1⦌))
          hs hs' =
        Cell.type₂ f c
Tactic: obtain rfl := SimplexCategory.eq_id_of_mono g
Cleared hypotheses: g, hs_1, hs, hs'_1, hs'