-- Lean 4 proof step from Mathlib.AlgebraicTopology.SimplexCategory.MorphismProperty (goal)
Goal:
d : ℕ
W : MorphismProperty (Truncated d)
inst : W.IsMultiplicative
δ_mem :
  ∀ (n : ℕ) (hn : n < d) (i : Fin (n + 2)),
    W (X := ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := n } ⋯) (Y :=
      ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := n + 1 } ⋯) (δ d i ⋯ ⋯)
σ_mem :
  ∀ (n : ℕ) (hn : n < d) (i : Fin (n + 1)),
    W (X := ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := n + 1 } ⋯) (Y :=
      ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := n } ⋯) (σ d i ⋯ ⋯)
c : ℕ
hc :
  ∀ (a : ℕ) (ha : a ≤ d) (b : ℕ) (hb : b ≤ d)
    (f :
      Quiver.Hom (V := Truncated d)
        (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := a } ha)
        (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := b } hb))
    (h : a + b = c), W f
a : ℕ
ha : a ≤ d
b : ℕ
hb : b ≤ d
f :
  Quiver.Hom (V := Truncated d)
    (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := a } ha)
    (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := b } hb)
h : a + b = c + 1
h₁ :
  ¬Function.Surjective (α :=
      Fin
        ((ObjectProperty.FullSubcategory.obj (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
              (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
                { len := a : SimplexCategory } ha)).len +
          1))
      (β :=
      Fin
        ((ObjectProperty.FullSubcategory.obj (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
              (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
                { len := b : SimplexCategory } hb)).len +
          1))
      (⇑(Hom.toOrderHom f.hom) :
        (a : Fin ({ obj := { len := a }, property := ha }.obj.len + 1)) →
          Fin ({ obj := { len := b }, property := hb }.obj.len + 1))
neg :
  ∀ (hb : 0 ≤ d)
    (f :
      Quiver.Hom (V := Truncated d)
        (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := a } ha)
        (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := 0 } hb))
    (h : a + 0 = c + 1)
    (h₁ :
      ¬Function.Surjective (α :=
          Fin
            ((ObjectProperty.FullSubcategory.obj (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
                  (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
                    { len := a : SimplexCategory } ha)).len +
              1))
          (β :=
          Fin
            ((ObjectProperty.FullSubcategory.obj (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
                  (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
                    { len := 0 : SimplexCategory } hb)).len +
              1))
          (⇑(Hom.toOrderHom f.hom) :
            (a : Fin ({ obj := { len := a }, property := ha }.obj.len + 1)) →
              Fin ({ obj := { len := 0 }, property := hb }.obj.len + 1))),
    W f
⊢ W f
have neg₁ : ∀ (b : ℕ) (hb : b + 1 ≤ d)
  (f :
    Quiver.Hom (V := Truncated d)
      (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := a } ha)
      (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d) { len := b + 1 } hb))
  (h : a + (b + 1) = c + 1)
  (h₁ :
    ¬Function.Surjective (α :=
        Fin
          ((ObjectProperty.FullSubcategory.obj (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
                (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
                  { len := a : SimplexCategory } ha)).len +
            1))
        (β :=
        Fin
          ((ObjectProperty.FullSubcategory.obj (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
                (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ d)
                  { len := b + 1 : SimplexCategory } hb)).len +
            1))
        (⇑(Hom.toOrderHom f.hom) :
          (a : Fin ({ obj := { len := a }, property := ha }.obj.len + 1)) →
            Fin ({ obj := { len := b + 1 }, property := hb }.obj.len + 1))),
  W f
Tactic: obtain _ | b := b
Cleared hypotheses: b, hb, f, h, h₁