∀ {X : AlgebraicGeometry.Scheme} (𝒰 : X.OpenCover) (x y z : 𝒰.I₀),
  CategoryTheory.Limits.HasPullback (CategoryTheory.Limits.pullback.fst (𝒰.f x) (𝒰.f y))
    (CategoryTheory.Limits.pullback.fst (𝒰.f x) (𝒰.f z))