∀ {X Y S T : AlgebraicGeometry.Scheme} {f : T ⟶ S} {g : Y ⟶ X} {iX : X ⟶ S} {iY : Y ⟶ T}
  (H : CategoryTheory.IsPullback g iY iX f) {US : S.Opens} {UT : T.Opens} {UX : X.Opens}
  (hUST : UT ≤ (TopologicalSpace.Opens.map f.base).obj US) (hUSX : UX ≤ (TopologicalSpace.Opens.map iX.base).obj US)
  {UY : Y.Opens} (hUY : UY = (TopologicalSpace.Opens.map g.base).obj UX ⊓ (TopologicalSpace.Opens.map iY.base).obj UT)
  [AlgebraicGeometry.Flat iX],
  AlgebraicGeometry.IsAffineOpen US →
    AlgebraicGeometry.IsAffineOpen UX →
      IsCompact ↑UT → IsQuasiSeparated ↑UT → CategoryTheory.IsIso (AlgebraicGeometry.pushoutSection H hUST hUSX hUY)