∀ {X Y : AlgebraicGeometry.Scheme} (f : X ⟶ Y) (𝒰 : Y.OpenCover)
  (𝒱 :
    (i : (CategoryTheory.Precoverage.ZeroHypercover.pullback₁ f 𝒰).I₀) →
      ((CategoryTheory.Precoverage.ZeroHypercover.pullback₁ f 𝒰).X i).OpenCover)
  (i : (AlgebraicGeometry.Scheme.Pullback.openCoverOfBase 𝒰 f f).I₀) (j : (𝒱 i).I₀),
  AlgebraicGeometry.IsOpenImmersion ((AlgebraicGeometry.Scheme.Pullback.diagonalCover f 𝒰 𝒱).f ⟨i, (j, j)⟩)