∀ {X Y S : AlgebraicGeometry.Scheme} {f : X ⟶ S} {g : Y ⟶ S} (T : AlgebraicGeometry.Scheme.Pullback.Triplet f g),
  ∃ t, (CategoryTheory.Limits.pullback.fst f g) t = T.x ∧ (CategoryTheory.Limits.pullback.snd f g) t = T.y