∀ {X Y Z : AlgebraicGeometry.Scheme} (𝒰 : X.OpenCover) (f : X ⟶ Z) (g : Y ⟶ Z)
  [inst : ∀ (i : 𝒰.I₀), CategoryTheory.Limits.HasPullback (CategoryTheory.CategoryStruct.comp (𝒰.f i) f) g] (x : 𝒰.I₀),
  CategoryTheory.IsIso
    (CategoryTheory.Limits.pullback.fst
      (CategoryTheory.CategoryStruct.comp
        (CategoryTheory.Limits.pullback.fst (CategoryTheory.CategoryStruct.comp (𝒰.f x) f) g) (𝒰.f x))
      (𝒰.f x))