∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] {sq₁ sq₂ : CategoryTheory.Square C} (self : sq₁.Hom sq₂)
  {Z : C} (h : sq₂.X₄ ⟶ Z),
  CategoryTheory.CategoryStruct.comp sq₁.f₂₄ (CategoryTheory.CategoryStruct.comp self.τ₄ h) =
    CategoryTheory.CategoryStruct.comp self.τ₂ (CategoryTheory.CategoryStruct.comp sq₂.f₂₄ h)